Incorporeal Movement. A Wraithheart has advantage on all Stealth checks when in dim or darker light. The Wraithheart can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.

Multiattack. The Wraithheart makes three Wraithclaw attacks. It can substitute its Crushing Futility attack for one of these.

Wraithclaw. Melee Weapon Attack: +7 to hit, reach 5 ft. Hit: 10 (1d10 + 4) slashing damage, plus 6 (1d10) necrotic damage.

Crushing Futility. The Wraithheart targets one creature it can see within 30 feet. The target must make a DC 15 Wisdom saving throw.

The first time a creature fails this save, it takes 7 (2d6) necrotic damage and has disadvantage on the next attack it makes. On a success, the creature takes half damage and does not suffer disadvantage on its next attacks.

The second time a creature fails this save, it takes 10 (3d6) necrotic damage and is Incapacitated until the end of its next turn. On a success, the creature takes half damage, is not incapacitated, and the saves it has made against this ability reset. 

The third time a creature fails this save, it becomes a Wraithheart. 

If a creature that has been targeted by this ability goes one minute without being targeted by it again (from any enemy), the saves it has made against this ability reset.