Armor of Agathys

From Player's Handbook, page 215.

Description
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 level above 1st.

Learned By

 * Classes: Warlock
 * Subclasses: Paladin (Oath of Conquest), Sorcerer (Clockwork Soul), Sorcerer (Giant Soul (Frost Giant))
 * Races: Levistus Tiefling, Mark of Warding Dwarf
 * Feats: Magic Initiate