Ebon Ray of Doom

You fire a ray of black energy that drastically slows down the rate at which the subject can recover hit points. A creature targeted by your ebon ray can't regain hit points by any means except healing magic. When the creature receives healing magic, it must make a Constitution saving throw against your spell save DC. On a failure, the creature regains no hit points.

When you target a creature with the Regeneration ability, the creature must succeed on a Constitution saving throw at the start of each of its turns before it regains hit points. Failure indicates that the ability does not function in that round.