Death's Door

You are spared from the clutches of death by an inch. 

The first time the target would drop to 0 hit points as a result of taking damage, the target instead drops to 1 hit point, and the leftover damage is reflected back onto the attacker.

The attacker must make a Constitution saving throw. If they fail, death's door is opened and a blast of necrotic damage reigns out equal to 3d8 + the leftover damage of the attack after the target has been reduced to a single hitpoint. If succesful, they take half damage. 

Any form of health reduction to 0, aside from ailgments, can be reflected by Death's Door.

If the target is at 1 hp, the spell has no effect.