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First and foremost, it should be said that in order to be spellscarred, you don't have to take any of these options.  Your spellscar can look like whatever you want (within reason) without taking a single power or feat.  The only requirements and effects of simply having a spellscar are those listed above.

Theme
Spellscarred Harbinger (I'd rate it sky blue, but I think that's my Bias speaking)
Level 1: pick a power that can be used once per encounter: Teleport 2 as a Minor, Reroll a failed Save, or turn invisible SonT.
Level 5: Daily that dazes a target you hit, and you take some damage
Level 10: pick a different power from the 1st level list, you can't use both in the same encounter however, still sometimes you don't need to reroll a save.

Optional Powers
Level 2 Utility: Plague Disruption - Attack penalty as an Aura matters little if you're at range, but is good if you're front line.  The recharge penalty is an interesting kicker.
Level 6 Utility: Torture Reality - Triggering off of being Targeted kinda sucks, especially if you're at range and not likely to have anyone next to you.  Still it gets you out of 1 attack/day if you're in the thick of it.
Level 10 Utility: Morphic Recovery - Few characters at this level (and even fewer once you hit Paragon) will be out of Encounter Powers before the 3rd round, and once you're out, you have to hit again to activate this.  Could work for characters with many non-standard attacks (like a Swordmage)

Except for the Theme, Spellscar powers can only be gained through the Multiclass feats (Novice, Adept, and Acolyte Power), and even have their own Paragon Multiclassing option (discussed later).  In order to qualify for those feats you have to take:
Student of the Plague - I'm not sure I can rate this, it's required to take any of the powers.  You get 3 options when choosing this feat:

• Sight of the Unseen:  1 Square Darkvision is pretty bad considering how easy it is to get light sources, and it doesn't help you 'sneak ahead' since if you're in the dark, chances are your opponents have real darkvision.  If it was Blindsight, it would be cool.
  


• Flamespeed:  Considering it costs a minor action, it's not that impressive, but it does last for 2 rounds, so I'm sure some builds can make use of it.
  


• Larger than Life:  A free action to gain +1 Reach has quite a few interesting combinations, from hitting a flying creature that thought they were safe, to Threatening Reach 5 (or more).

(W) indicates a Weapon Keyword attack power
(I) indicates an Implement Keyword attack power
(N) indicates an attack power without an Accessory keyword

***Author's Note: Most of these powers are Bad or Worse simply comparing them to what I thought a power of that level should be like, what I did not take into account is the Feat Cost involved with getting the power.  Because taking Student of the Plague counts as your Multiclass choice and each of these powers take an additional Feat to Replace (bolded for emphasis) one of your current powers, you should really only consider any of these an option for Fluff or very specific Combo purposes (such as the old Stinking Cloud + Blood Pulse + Gravity of Moment).  I honestly believe the entire thing needs to be rewritten as a Hybrid selection, given the time, I might end up doing that for people to use in Home games.


Level 1 Encounter
Horrific Maw Bite (N) - Lets face it, a non-weapon melee attack with +2/4/6 scaling isn't worth taking, much less spending a feat on.
Plague Lash (I) - The only saving grace of this power is that it has 2 keywords and one of them is Psychic.  Restricting the bonus damage to only attacks on their next turn really sucked.

Level 1 Daily
Burning Focus (W) - A Level 1 Multi-attack power that you don't really need to hit with?  Very nice.  The fact the Effect is all Creatures Adjacent makes this a bit of a double edged sword, but combining it with grouping abilities is fantastic.
Spellmirror (I) - I'm not fond of immediates triggering off you getting missed, still, the damage is good for an off-turn attack, the range is impressive, and it works well against status causing enemies.

Level 2 Utility
Spelleater - +4 against an attack targeting your NADs isn't bad, and free Temps assuming they miss is great.  The fact the attack might still hit, or would have missed anyway keeps this from being better.
Plague of the Rooted Majesty - Reducing your speed by 2 is pretty painful to gain a Dwarf ability considering Dwarves are speed 5.  Of course, most battles only involve significant movement in the first couple rounds.  Immune to Prone, and having a Climb Speed can both be useful.  If you can teleport regularly or have other movement modes (Mounts, Shift X) spending a feat and a minor action to become immune to Prone and reduce forced movement is almost a must.

Level 3 Encounter
Scar-Carving Blade (W) - Even though it has a higher potential damage die, the effect is worse than Plague Lash.  Every class has something better than this.
Call of the Plague (I) - It's a lot like Come and Get it for Implement users, but it's unfriendly, you have to hit first, the pull is random, the damage is higher, and then you get to push them a random distance too.  I want to rate this higher, but the unpredictability keeps it lower.

Level 5 Daily
Blurring Blade (W) - They'll get to save against the ongoing before taking any of the damage.  I could see using it with the feat that deals damage when an enemy fails or succedes on a save though.
Terrible Imposition (N) - Even though you can't be attacked for a round, it's still a +2/4/6 attack bonus, it only does at most 20 damage, and you have no choice but to attack it and only it.

Level 6 Utility
Plaguefire Body - Ending a Charm or Fear effect is actually pretty cool
Haunting Reception - This would be great for anyone that tends to draw attacks other than a Defender, if only it weren't a Daily

Level 7 Encounter
Cursed Grasping Air (I) - Technically the slide doesn't have a maximum, so one enemy can be removed from the battlefield or ping ponged to death in a zone.  Not that Great if your DM looks at you when you try this and says "No."
Reach of the Doomed (W) - A Reach 3 Double Attack that Dazes, and the 2nd attack is against Fort too!  Well worth spending the Surge for most characters that could take this.
Venomous Bloodfang (W) - Necrotic damage sucks, but you'll still do some damage, and you'll still heal a decent amount.  Many classes have better self-healing powers though.

Level 9 "Daily"
Death Vortex (I) - Yes, that's an Encounter Power, but lacking an accessory keyword makes it not really worth the action, much less the sustain. At least you can sack it for other powers with the Savant feature?
Burning the Plagued Bellows (I) - This is the Daily version of Death Vortex, the only real differences are Blast 3, and the Sustain is (save ends) instead of spending a Healing Surge, all in all that makes it weaker.
Submission of the Earth (W) - A 3[w] single target Daily?  What is this, 1st level?

Level 10 Utility
Tears of Fire and Blood - It's difficult to get a large attack bonus for yourself, and the fact the damage you take is Fire makes it absurdly easy to resist by this level.
Scar of Blue Wings - Poorly Worded, if this is supposed to be a Free Action with Trigger: You take a Move Action, then it's probably Great.

Level 13 Encounter
Lines in Burning Sand (I) - Implement vs AC is bad, but an Auto Damage Roll that can be abused with forced movement is amazing on an Encounter power if you can take advantage of it.
Spellmother's Embrace (N) - Worse than simply having a Garotte.

Level 15 Daily
Infliction of the Flamemist (I) - Giving your enemy Insubstantial is a bad thing unless you and your allies have a substantial number of Fire based attacks, in which case this is Great.
Rabid Chain of Steel (W) - Putting my disdain for MBA forcing aside, this power needs to be reworded to "... not been attacked by this power this turn."  Currently it can't be combined with gathering powers which makes it reliant on your allies.

Level 16 Utility
Body, No Body - "Until the end of this turn" really sucks, slightly better if you gain additional benefits from being Insubstantial.
Healing Flesh of the Infected - Surgeless Healing + Temps for an ally at the cost of your Surge, sadly it's a Standard Action with no Healing Keyword so there's very few combo's to pull off.

Level 17 Encounter
Iron Tooth of Bloodlust (W) - 1[w] higher than Venomous Bloodfang, I'd expect more for 10 levels higher.
Mask of Midnight (I) - Most classes that would take this have better control powers at this level, but that doesn't stop Blast 4 Psychic Blinding from being good.

Level 19 Daily
Gravity of Moment (I) - Sliding 1 target 15 squares is not significant on it's own.  However, I'm tempted to rate this Very Good because of how many ways there are to abuse Slide 15 (10 on Miss) with Sustain Minor Slide 10, with other powers/feats this can very well 1-shot anything.
Spellcurse (W) - I'm fairly certain there's Encounter powers that do this at level 3.  Stacking Dazed ontop of Immobilized is just weird.

Level 22 Utility
Donning the Mudflesh - Insubstantial until you're Bloodied as a Utility Stance is pretty neat, Monks have a better version though.  Better if you take advantage of Insubstantial.  Worse if you're a class with too many stances already.
Spellplague Guardians - For classes like Fighters that lack the zones and walls that controllers and some others get, this can be a pretty good control power to block off melee.  Especially since it's movable but doesn't require a sustain action.

Level 23 Encounter
Hideous Tether (W) - It doesn't do weapon damage which is just weird, the effect is frankly worse than Immobilize but Ongoing from an Encounter Power is fairly rare.
Tongues of the Earth (I) - Again, there's better powers in Heroic.

Level 25 Daily
Contagion Flame (W) - Again no weapon damage?  Unless you move away from the target, you gain the Ongoing.  I'm sure that some gathering combo's can make this Better, but it's party unfriendly (save ends) even if it is resistable.
Malignant Growth (I) - I'm really not sure how to rate this because it doesn't really say what the mouths do.  The points of debate are what defense they attack, and if you get to add your damage bonuses to the 1d10.  If you can convince your DM that they attack Fort and you get to add your Damage, then this power creates your own personal Twin Striker.

Level 27 Encounter
Impossible Flurry (W) - Oh, I get it, these weapon powers with no weapon damage are so non-monks can pretend to be monks!  Sadly, Monks, and most other classes have much better powers; friendly Burst 3 or not I expect some real status effects in Epic.
Shifting Wave (I) - Kinda like this.  Same range, targets will, better damage that's part Psychic, and very nice battlefield shifting with abusable Teleporting.

Level 29 Daily
Echoes of Valor (W) - I really want to rate this Gold, but every class will have sufficient amazing powers by this level to make it not a must have.  Reliable with Big damage, sustain minor to repeat the attack until you miss and you don't have to stay in reach.  This attack will be remembered, because it repeats itself.
Metamorphosis of Spellflame (I) - Implement attack but Weapon range? What the hell.  Fairly low damage, the only way to make this halfway decent is a build focused on ongoing damage and Save Penalties.

Paragon Path
Spellscarred Savant
Plague Familiarity: The level 11 Feature reverses the Spellscarred Susceptibility into +2 bonuses which is necessary, but not particularly strong in any game that doesn't feature  Plaguechanged/Spellplague.

Spellfire Action: The AP Feature is fairly worthless.  If you're spending an AP for anything other than an extra action it needs to be a better bonus than regaining a power.  It does synergize with the level 16 feature, but still.

Plague Mastery: Because this is like a Paragon Multiclass you get to choose a level 7 or lower Encounter power at 11th, and a level 19 or lower Daily power at 20th.  If you don't care about your PP, this is a better way of gaining 2 powers than spending feats for them

Spellfire Healing: Your Utility power is an Encounter power that lets you spend a Surge when you take Fire Damage (And ends ongoing fire if that's what triggered it).  Like most spellscarred powers, there's builds that can make use of this, but for the most part it will be useless.

Spellfire Mastery: The level 16 Feature is a real gem for Arcane Classes, it allows you to switch any Spellscarred Encounter Power to regain an Arcane Encounter Power (once per power).  Without any additional Feat expenditure, you have 2 Spellscarred Encounter Powers already, and can even choose that Daily Encounter Power at level 20.  Since there's no level requirement on which powers you can regain, for a 20th level Arcane user you can regain whatever Encounter power you like.

FEATS

Most of these feats have an added benefit for also having the Student of the Plague feat, I will give a separate rating for Students.

Arcane Malice - A small damage bonus against a single enemy that you hit with a daily excludes this to only really being good for Solo's or Nova builds.  Student's don't have it significantly better.

Corrupting Attacks - I like to think of this as adding "Enemies you hit take a -1 penalty to (Defense) until the end of your next turn" To all your Encounter/Daily Powers.  It's not hard to take advantage of this, especially if you're targeting AC.  It's better if you are specialized along with your Party.  Students get double the effect which can make this feat fantastic.

Pernicious Onslaught - It can work well with a build focused on Ongoing Damage and Grouping effects (Tiefling Pyro).  It doesn't get much better for Students.

Ravenous Blessing - It's a healing tradeoff of 1/2 Level - 3 so taking it before Level 7 is next to worthless, however at level 30 it's +12 to the Party's HP per use (You take 18, Ally heals 30).  Works particularly well for Leaders that give THP to themselves.  The Student benefit adds nicely to any leader.

Earth Mote's Stability - With a good enough Athletics (DC 30 trivial for a Paragon Goliath) this is +1 speed.  It also allows you to jump 3 while Slowed.  The falling resistance is ok.  Students might want to jump 4 while slowed and never fall off anything, even if they can't manage a DC 35 athletics.

Flamespeed Shifting - Shift 2 is very useful for obvious reasons.  Students that are strikers will love this, almost makes Razorclaw better than Longtooth.  Keep in mind this is bonus to damage, not damage rolls, so it will add the fire keyword and 2 damage to things like Magic Missile, Beguiling Strands, and Punishing Eye to name a few.

Lingering Fade - I'm sure some Gnome's could make use of Sustain Standard Invisibility though I can't think of anything worth it, especially with the damage.  Being a Student just reduces the damage you take.

Memories of Destruction - I just don't see using up your Deva racial as an Interrupt to make a save against Daze/Stun (save ends) as worth a feat.  The Student benefit is trivial.

Path of the Scarred - +1 Fire damage (not just to damage rolls) for a Humans Encounter and Daily powers is quite nice, but not for every class.  Students get a +2 which isn't amazingly better.

Plague of Chance - This suffers from Dazing Rebuke Syndrome, it's good for CA but 1/encounter conditional CA isn't worth a feat for Halflings.  Student benefit isn't really better.

Plagued Breath - I'm not a fan of adding the highly resisted Necrotic keyword, though this is a cheaper and earlier way of getting a second keyword on Dragons Breath.  I can see an Ancient Soul making good use of this.  Students get to add ongoing necrotic, which Ancient Souls can certainly use to great effect.

Plagued Regeneration - A small scaling increase to a Longtooths regeneration is neat, but you deal unfriendly Necrotic damage to adjacent creatures.  The benefit for Student's is a joke.

Plagueforged Accuracy - Ignoring Cover and Concealment with Elven Accuracy can be useful, you also add 2 keywords.  Students get a small damage burst.

Plaguing Bloodhunt - 3/6/9 Necrotic damage when you hit bloodied enemies is pretty nice for almost any Tiefling (I'm actually not sure why so many guides rate similar powers so poorly).  Students add Fire to the type of damage.  Note that this feat deals damage even if the power doesn't, the only requirement is hitting.

Planar Blurring - Yet another feat that lets you do something when you Fey Step, and Slide 1 is fairly weak, the only real benefit is that it's any creature adjacent and not just allies.  Students get to teleport a second target 2.

Poison Plague Feast - Ongoing poison doesn't have enough of a presence to make this good for Dwarves.  The Student benefit doesn't change this

Scar and Fury - More adjacent creatures nonsense and 2 damage is just weak for a Half Orc, useful for minion killing I guess.  Students do 3 more damage, woo.

Scar of the Shadowweave - Adds cold and 2 more damage to Reaping.  Students gain concealment.

Spellcharged Channel - When you use your Channel Divinity, you get to automatically Debuff an adjacent enemy, or Buff an adjacent ally.  Students get to do both, but I don't see that as raising the rating.  Better if you can use multiple CD's per encounter.

Spellscarred Invigoration - This is getting black just for it's vs Minion use.  Obviously you don't take this if you can't Mark.  Defender Students can get some pretty good healing out of this.

Spellscarred Subterfuge - +2 Feat bonus to Diplomacy/Bluff?  Those are more common than cheese.  What Half-Elf disguises their spellscar anyway?  Being a Student makes this feat at least a better choice than Skill focus.

Tainted Darkness - +2 Fire damage every time your Darkfire target get's attacked doesn't seem like much, but it's a very nice nova enabling thing.  Students give their allies concealment from the target, which isn't that significant since they're just going to die this round anyway.

Tenacity for Living - Primal characters don't burn or decay as well?  Those damage types are actually fairly common for ongoing, I just don't see +4 to saves particularly worth a feat.  Maybe if you're a Student and have lots of ongoing fire/necrotic powers, giving your enemies a penalty to save can be good.

Unexpected Flourish - Gaining an Arcane At-Will as an Encounter when you're a Martial class is finally possibly useful as of HotFey, thanks to MC Skald for the Aura and picking one of their at-will powers, not many Martial classes that use BA have a use for 13 Cha though. Students get to use their best stat if they decide to go with something requiring it... Keep in mind that this doesn't grant you the ability to use Implements

Reserved for Builds

This being my first guide, suggestions are welcomed so long as you agree with my opinions.

Maybe I'm looking at sight of the unseen wrong, however by RAW, you're able to see behind you at all times with that power running.  So....no one could sneak up on you.  Pretty useful in some non-combat situations.

I'm not sure how that differs from carrying a light source or, for those wanting to keep a low profile, the Dark Light ritual

@Backlash: True, I hadn't even mentioned the level 16 Feature or given it much thought, thanks.  That basically turns the Level 7 Encounter power (and the Daily if you choose Death Vortex) you have to choose into additional uses of any other Encounter power, regardless of Level.  Hello Prismatic Burst.  I'll add a note about rating change.

Larger than Life is terrible, and only black because the other options are so terrible as well. +1 reach is great. Getting it once per day is meaningless. Making it cost your multiclass feat turns it bright red.

Spellplague Guardians is also pretty good. The guardians form an unbreakable wall five squares wide (that you can move) that allies can move and attack through, but enemies cannot pass through and grant cover for shooting through. In tight quarters, this is basically a one-power-win against melee opponents. They can also be used to trap melee opponents and take them out of combat without a save.

Oh I agree that Larger than Life is entirely unworthy of a feat, my stated assumption is that you've already decided to MC into Spellscarred, and thus of the 3 options it's by far the best.  I couldn't exactly create a guide with anything other than Red through Black otherwise, since without specific combo's none of these powers are worth 2 feats.

With a running start it's a DC 15 to jump over the guardians.  At level 22 every enemy passes that on a 4 minimum, assuming they can't fly, teleport, earthglide, or phase.  Allies also can't stand in those squares, so an enemy with reach can stand adjacent and your melee allies have to wait for you to spend a move action for them to attack.

I will admit that I'm just comparing it to the Epicness of Illusory Wall and other conjurer powers, specifically: Clerics, Druids, Seekers, Shamans, Invokers, Psions, Wardens, and even Rangers all have comparable or better choices at earlier levels, and that's just off the top of my head.  I will change it to black and note the blocking potential.

@Damiel: I'll add it, thanks.

@Herid: Good point, Plate Armor doesn't make you speed 1 while slowed, so while there's no RAW about speed penalties, there's certainly logical precedent.  And now that I think about it, for my Swordmage with Teleport 5 at-will, this is spending 2 feats to become immune to prone and reduce forced movement, definitely something I need to think about.

It's even better than that, because it can be an ally's power too- it's possible to give several party members a second use of their best encounter power regularly if you have some arcane allies.

Wow, I actually missed that part. That is indeed excellent. The first combination to come to mind is a Battlemind (to cash in on Spellscarred Con-based weapon attacks) providing support to an arcane striker or leader.

"I'll hold them down, you finish them off/keep me alive with this."

Yikes!  That actually does open up a lot of possibilities.  This gives me an idea for a Duo of Dr Frankenstein and Igor.

Updated with Theme.