Anders's Revoke Immortality

You attempt to temporarily suppress the essence of a creature's immortality. Choose one celestial, fiend, or undead within range. The target must make a Constitution saving throw or be affected by this spell for the duration. While affected by this spell, the creature suffers the following effects:

• If the target has resistance or immunity to poison or necrotic damage, it loses them.
• The target cannot regain hit points.
• When you cast a spell that deals poison or necrotic damage or that is of the necromancy school that requires you to make an attack roll against the target, you have advantage on the attack roll, and when you cast a spell that deals poison or necrotic damage or that is of the necromancy school that requires the target to make a saving throw, it has disadvantage on the saving throw.
• Whenever the target takes poison or necrotic damage from one of your spells, it takes additional damage equal to your spellcasting ability modifier.

Whenever the target attempts to regain hit points, it can make another Constitution saving throw against this spell, ending the effect on a success.