Warlock Attack 1Armor of Agathys

You surround yourself in a sheath of black ice from a dark and doleful realm. It protects you from attack and radiates fierce cold.

Daily        Arcane, Cold
Standard Action      Personal

Effect: You gain temporary hit points equal to 10 + your Intelligence modifier. Until the end of the encounter, an enemy that starts its turn adjacent to you takes 1d6 + Constitution modifier cold damage.

Published in Player's Handbook, page(s) 132, Class Compendium.