Cerbursine

Keen Sight and Smell. The cerbursine has advantage on Wisdom (Perception) checks that rely on sight or smell.

Legendary Resistances (4/Day):  If the cerbursine fails a saving throw, it can choose to succeed instead.

Multiple Heads: The cerbursine has three heads that can attack independently or together. However, should the cerbursine take 50 or more points of damage in a single turn, one of the heads will die. If all heads are killed before the cerbursine's next turn, the cerbursine will have disadvantage on all attacks and saving throws until it dies. 

Regeneration. The cerbursine regains 20 hit points at the start of its turn. If the cerbursine loses one of it's heads or takes fire damage, this trait doesn't function at the start of the cerbursine's next turn. The cerbursine dies only if it starts its turn with 0 hit points and doesn't regenerate.

Immutable Form. The cerbursine is immune to any spell or effect that would alter its form.

Magic Resistance. The cerbursine has advantage on saving throws against spells and other magical effects.

Magic Weapons. The cerbursine’s weapon attacks are magical.

Multi-attack. The cerbursine makes three attacks: one with one of its heads and two with its claws or three bite attacks, one with each head.

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one creature. Hit: 10 (1d8 + 5); piercing damage.

Claws. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 14 (1d10 + 5 slashing damage:

Psionic Howl: The cerbursine lets out a loud and a damaging howl from all of its heads causing all those within range to tremble. Creatures within 60ft must make a Wisdom Saving Throw (DC 17). On a fail, the creatures take 3d8 Psychic damage (one for each head still alive) or take half as much on a success. 

The cerbursine can take 4 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The cerbursine regains spent legendary actions at the start of its turn.

Claws. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 14 (1d10 + 5) slashing damage.

Triple Bite (3 Actions). Melee Weapon Attack: +10 to hit, reach 5 ft., one creature. Hit: 24 (3d8 + 5); piercing damage.