When you take acid, cold, fire, lightning, or thunder damage, you may use a reaction to capture that element. You have resistance to the triggering damage type immediately and until the start as your next turn.
The first time you hit with a melee attack on your next turn, the target takes an extra 1d6 damage of the absorbed type. This damage scales normally.
Sometimes, he ward gets overloaded and leaks. Naturally, Archmage Kottar decided this was a feature and standardized it.
-Abjurist Ovtuch Irieya