A lashing tail of lightning protects you from a foe that thinks it has the advantage.

Encounter   ✦     Arcane, Implement, Lightning
Immediate Reaction      Melee 1

Trigger: An enemy moves into a space where it flanks you

Target: The triggering creature

Attack: Charisma vs. Reflex

Hit: 2d10 + Charisma modifier lightning damage.

  Dragon Magic: You push the target a number of squares equal to your Strength modifier.

Published in Player's Handbook 2, page(s) 144.