Wendigo

Spellcasting. The wendigo is a 9-level spellcaster. Its spellcasting ability is intelligence (spell save DC 17, +4 to hit with spell attacks) The wendigo has following Druid & wizard spells prepared:
Cantrips  (at will):  Primal Savagery, Poison Spray, Druidcraft 
1st level  (5 slots): Entangle, Fog Cloud, Snare                                         
2nd level (4 slots): Maddening Whispers, Spike Growth, Misty Step
3rd level  (4 slots): Orr's Mark of Fate, shadow portal, bestow curse                                            4th level  (4 slots): Shadow Step, Lunar Transfer, Guardian of Nature                                          5th level  (5 slots): Feast of flesh, Krail's Rot, Living Shadow

Sense magic.  The wendigo senses magic within 120 feet of it at will.  This trait otherwise works just as detect magic.

Devils sight.  Magical darkness does not impede the wendigo's Darkvision.  

Shapechanger.  The wendigo can use its action to polymorph into a small or medium creature it has seen in the last 72 hours, or back into its true form.  Its statistics stay the same (other than its size) in each form.  

Supernatural Being.  If the wendigo's body is slain or destroyed, its spirit can attempt to possess any humanoid  with half, or less than half of their hit points.  The target must succeed a DC 24 constitution saving throw, or become possessed.  If the possession takes hold, it will take 1d12 days before they are completely transformed into a wendigo themselves.  This can only be stopped by the banishment spell.  

Lasting thought.  After a creature's first encounter with a wendigo, they become terrified. The wendigo haunts the creature's dreams, making them have a -4 on all attack rolls for 1D8 days, and have a long rest only function as a short rest. If they want the effect of a long rest, they must take two. Any time the creature enters a place unfamiliar to them, they must make a DC 16 wisdom saving throw, or see the wendigo within 30 feet of them. The illusion can't affect anything around it, but the creature. It can only reduce the creature to half health if it attacks, or the creature suffers 2D12 psychic damage, which can't be healed until after the next long rest, or dawn (whichever comes first)

Vocal mimicry.  The wendigo can perfectly mimic any noise or voice that it has heard in the past 24 hours.

Horrifying Visage.  The Wendigo targets one frightened creature it can see within 60 feet of it. If the target can see the Wendigo, it must succeed on a DC 15 wisdom saving throw against this magic or drop to half it's hit points.

Unyielding Hunger.  The wendigo can consume a kill it has made in the last 48 hours, and regain all of its hit points instantly.

Multiattack.  The wendigo can make two claw attacks.  two at one target, or one at two targets if they are within 10 feet of each other.

Claw. Melee Weapon Attack: +7 to hit, reach 10 ft., 1 target. Hit: 11 (2d6 + 4) slashing damage. 

Gore. Melee Weapon Attack: +8 to hit, reach 5 ft., 1 target. Hit: 15 (2d10 + 10) bludgeoning damage. 

Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., 1 target. Hit: 9 (2d6 + 2) piercing damage. 

Antlers. Melee Weapon Attack: The wendigo charges recklessly at one target (or two that are standing next to each other).  The creature(s) must succeed a DC 16 strength saving throw, or be pushed 30 feet away falling prone, and taking 2d8 + 10 bludgeoning damage on a failed save.  On a successful save, they are pushed 15 feet away, and take half the damage. 

Echoing Wail. The wendigo releases a horrifying scream after an attack is successful on it.  Any creature within 60 feet of the wendigo must succeed a wisdom saving throw or have disadvantage on their next attack on the wendigo.