Multiattack

The undead phoenix makes three attacks: two with its claws and one with its decaying bite.

Claws

Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 20 (4d6 + 6) slashing damage.

Decaying Bite

Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 15 (2d8 + 6) piercing damage plus 14 (4d6) necrotic damage. The target must succeed on a DC 15 Constitution saving throw or be cursed with perpetual decay. The cursed target can't regain hp until the curse is lifted by the remove curse spell or similar magic.