Armor of Agathys

Source: Player’s Handbook

1st-level abjuration

Casting Time: 1 action
Range: Self
Components: V, S, M (a cup of water)
Duration: 1 hour

A protective magical force surrounds you, manifesting as a spectral frost that covers you and your gear. You gain 5 temporary hit points for the duration. If a creature hits you with a melee attack while you have these hit points, the creature takes 5 cold damage.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, both the temporary hit points and the cold damage increase by 5 for each slot.

Classes: Warlock