An enemy assails you with elemental power, and you absorb it and use it to strengthen your own attacks.

Encounter  
Immediate Reaction      Personal

Trigger: Your resistance to fire, lightning, or cold damage prevents damage to you.

Effect: Once before the end of the encounter, you can deal damage of the same type (of one of those three types) equal to your resistance value. You must choose to gain this bonus to damage after the result of the attack roll is known, but before damage is dealt.

Published in Forgotten Realms Player's Guide, page(s) 53.