Death Ward

4th-Level Abjuration Spell (Cleric, Paladin)

Casting Time: Action
Range: Touch
Components: V, S
Duration: 8 hours

You touch a creature and grant it a measure of protection from death. The first time the target would drop to 0

Hit Points (HP) are a measure of how difficult it is to kill or destroy a creature or an object. Damage reduces Hit Points, and healing restores them. You can't have more Hit Points than your Hit Point maximum, and you can't have less than 0. See also Breaking Objects and Damage and Healing.

before the spell ends, the target instead drops to 1

Hit Points (HP) are a measure of how difficult it is to kill or destroy a creature or an object. Damage reduces Hit Points, and healing restores them. You can't have more Hit Points than your Hit Point maximum, and you can't have less than 0. See also Breaking Objects and Damage and Healing.

, and the spell ends.

If the spell is still in effect when the target is subjected to an effect that would kill it instantly without dealing damage, that effect is negated against the target, and the spell ends.

Source: Player's Handbook, p. 261