Death's Door

Choose a humanoid you can see and make a spell attack, this is neither ranged nor melee, if you succeed reduce the target to 1 hit point. If the target is hit with this effect it cannot be reduced below 1 hit point until your next turn or until it restores hit points. If you fail to hit the spell, your hit points are reduced to 1 instead and until the end of your turn you cannot gain hit points in any way.

A creature immune to necrotic damage will result in an automatic failure, while resistance gives you disadvantage. Even if you're resistant or immune to necrotic damage, on a fail your hit points will be reduced to 1.