Possession - The Wendigo begins to plant a part of its essence into a target that it can see.  Any wisdom ability checks and saving throws that target makes are made with Disadvantage and if they fail their roll by 5 or more, they take 20(4d10) psychic damage.  Whenever the target targets the Wendigo for an attack, they must make a successful saving throw against your Spell Saving Throw that, on a fail, causes the attack to fail or miss.  If the save to attack is failed by 5 or more, they attack themselves as they become overtaken by the Wendigo Spirit and gain the Frightened condition.  This effect can only be used on three targets at a time, and can be applied or redirected as a bonus action.

Glimpse of Humanity - a Warlock transformed into this creature can cast spells in this form using their Spell Casting abilities as normal.

Icesheet (2-4 Rounds).  As an Action:  the Wendigo can siphon the energy within a 60-foot cone ahead to freeze all targets within.  Creatures in the cone must make a constitution saving throw and on a fail, the cone deals 72(12d12) and are incapacitated as they are frozen in the ice.  On a success, they take half damage and are knocked prone.

Multi-Attack.  The Horror can make Five attacks:  Three with either its Hooves, Ice Pillar or the Staff and Two with its Bite attack.

Hooves.  Melee Weapon Attack: +12 to hit, reach 5ft, Hit: 36(6d10+6) Bludgeoning Damage

Staff of the Cold Stag.  Melee Weapon Attack: +10 to hit, reach 20ft, Hit:  46(8d10+6) Cold Damage

Bite. Melee Weapon Attack: +12 to hit, reach 10ft, Hit: 25(4d10+6) Piercing Damage.  After hitting, you regain a number of Hit Points equal to the damage dealt.

Ice Crystal.  Ranged Spell Attack:  +10 to hit, range (120/480ft), Hit: 45 (10d8+5) Cold Damage

Ice Prison.  A 20-foot cube area with water or snow in it can be contaminated by magic to set up a trap that, when entered by a Hostile target, releases pillars of ice that wrap around the target, forcing a DEX 20 saving throw that, on a fail, reduces the target's movement to 0 until they either break out by destroying the pillars (30 HP, 18 AC), are set free as an action by another creature or successfully make the save at the end of their turn.

Frostbite.  When you are hit by a melee attack, debilitating frost falls off of the bone dealing 30 Cold damage to any targets within 5 feet of you and reduce their speed by 10 feet.

Opportunity Attack - make two attacks on the target.

Misty Step - Teleport up to 30 feet away, causing the attack you can see miss.

Ethereal Step (1 Bonus Action, Once per Minute).  After making a Hooves attack, the greater Wendigo can teleport instantly to a tree it can see within 120 feet and become invisible. 

Lord of Fear (Recharge 4-6 rounds).  If there are any humanoids available nearby, you can create Wendigo horrors to possess the bodies, creating up to two lesser and two greater Wendigos.