Arcane Admixture

Paragon Tier
Prerequisite: 11th level, any arcane class
Benefit: Choose one arcane power that you know, and choose acid, cold, fire, lightning, or thunder. The power now deals that damage type in addition to its normal damage type and gains that keyword.
Each time you gain a level, you can choose to reassign either the power to which this feat’s effect applies or the additional damage type applied (or both).
Special: You can take this feat more than once. Each time you select this feat, choose a different power. You can’t apply this feat to the same power more than once.

Published in Arcane Power, page(s) 130.