Bonebrand

You sear an undead creature with radiant energy, branding its very soul. The creature takes 3d8 radiant damage and cannot regain hit points for the duration. On each of your turns, you can use a bonus action to deal an additional 1d8 radiant damage. If the creature succeeds on the saving throw it only takes half of the damage.

Undead have disadvantage on the CON saving throw.