D&D4 Wiki
Armor of Agathys
warlock (infernal) attack 1

Daily ✦ arcane, cold

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."[PH:132][U :W]

Armor of Agathys is a daily power available to warlocks at 1st level.