Wendigo

Unnerving Gaze. Every turn that a creature looks at the Wendigo's eyes, they must succeed a DC16 wisdom saving throw or be frightened for as long as the creature is within 40 ft of the Wendigo. 

Barbaric Possession. If the Wendigo focuses on an unconscious target for 1d6 hours, it will possess the victim. While the entity is possessed, they will obey the Wendigo's instructions, their alignment changes to chaotic evil and they gain a taste for the flesh of their respective species. This has no effect on other undead, celestials or constructs.

Spellcasting. The Wendigo is a 9-level spellcaster. Its spellcasting ability is intelligence (spell save DC 13, +7 to hit with spell attacks). The Wendigo has following wizard spells prepared:
9th level (3 slots): Weird

Phantasmal Slash. Melee Weapon Attack: +12 to hit, reach 6 ft., 1 target. Hit: 13 (2d12 + 2) necrotic damage. If the target does not succeed a DC15 wisdom saving throw, the sheer terror they experience will cause them to fall unconscious.

Psychotic Brainwave. Ranged Weapon Attack: +20 to hit, range 40/45 ft., 3 targets. Hit: 13 (2d6 + 3) phsycic damage.

Due to being an ethereal, evil spirit, Wendigos don't have full power in the mortal realm. They have 5 charges to spend on legendary actions. They regain 1d4 + 1 charge at dawn.

Spectral Haze (2 charges). The Wendigo gathers ghostly fog around it and disappears into the mist, becoming invisible. The effect ends when they perform an attack or cast a spell.

Bone Crunch (2 charges). Wendigos also have the ability to stare at their targets in such a way that their bones snap from the pure strain and terror inflicted on them. The target must succeed a DC14 constitution saving throw or take 1d20 bludgeoning damage and become incapacitated.