Death Wish

Choose a willing creature that you can see within range that has less than 1/4th of their maximum HP. Until the spell ends, the target gains the feature undead fortitude (described in this spell), it has advantage on Constitution saving throws, and the first attack the target makes that successfully hits on a turn becomes a critical hit.

Undead Fortitude. If damage reduces the target to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the target drops to 1 hit point instead.

The spell ends early if the target is reduced to 0 hit points or if they ever have more than half their hit points. When the spell ends, the target drops unconscious, immediately fails a death saving throw, and takes one point of exhaustion.