Borrowed Time

Target must be a creature at 0 HP but not dead. You draw upon a targets remaining life force to temporarily invigorate them. If the target has hit die remaining, the spell expends 1 hit die and gains temporary hit points equal to the maximum value of the die for the duration of the spell. If the spell ends before their actual hit points are replenished above 0, they make a death save. On a success they drop to stable, on a failure they get 2 failures. A natural 1 would be a 3rd failure and the target dies, their life force drained too far to continue on the mortal coil.

At 5th level increase the number of hit die up to 2 if they have them remaining. At 11th up to 3, at 17th level up to 4.