Wendigo Spirit

Ethereal Sight. The spirit can see 60 feet into the Ethereal Plane when it is on the Material Plane, and vice versa.

Incorporeal Movement. The spirit can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.

Frozen Heart. When it drops to 0 hit points, the wendigo (possessed creature) doesn't outright die. While it has 0 hit points, the wendigo cannot be destroyed or killed. Instead, if the body of the wendigo is left to the elements, it will continue to regain strength and continue living. While at 0 hitpoints, the wendigo's hitpoint maximum is 0. This number increases by 5 every day until it reaches its original maximum hitpoint count. 

To prevent this supernatural revitalization, one must tear the heart from the wendigo's body. Anyone who touches this heart must make a DC 16 Constitution saving throw, taking 22 (5d8) cold damage every round they touch it on a failed save, or half as much on a successful one. The heart must then be placed in the center of an ongoing fire for 24 hours until the heart melts away, removing and destroying the wendigo spirit. 

Alternatively, the spirit may be taken out and destroyed by any Druid or Nature Domain Cleric conducting a ritual. This ritual takes one hour to complete, but has components (listed below).

• One silver dagger worth at least 15 gp.
• A sacred wooden mask for everyone present.
• A drum being played throughout the ritual.
• An ongoing fire no smaller than 1 cubic foot (consumed by ritual).
• 30 gp worth of incense (consumed by ritual).
• One 5th level spell slot from anyone present (consumed by ritual).

Multiattack. The spirit makes two Leeching Touch attacks on its turn. 

Leeching Touch. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 17 (4d6 + 3) necrotic damage and the spirit heals an amount equal to half the damage dealt. 

Etherealness. The spirit enters the Ethereal Plane from the Material Plane, or vice versa. It is visible on the Material Plane while it is in the Border Ethereal, and vice versa, yet it can't affect or be affected by anything on the other plane.

Horrifying Visage. Each non-undead creature within 60 feet of the spirit that can see it must succeed on a DC 13 Wisdom saving throw or be frightened for 1 minute. If the save fails by 5 or more, the target also ages 1d4 × 10 years. A frightened target can repeat the saving throw at the end of each of its turns, ending the frightened condition on itself on a success. If a target's saving throw is successful or the effect ends for it, the target is immune to this spirit's Horrifying Visage for the next 24 hours. The aging effect can be reversed with a greater restoration spell, but only within 24 hours of it occurring.

Wendigo Possession(1). One humanoid that has consumed human flesh that the spirit can see within 15 feet of it must succeed on a DC 20 Charisma saving throw or be possessed by the spirit; the spirit then disappears, and the target is incapacitated and loses control of its body. The spirit now controls the body but doesn't deprive the target of awareness. The spirit can't be targeted by any attack, spell, or other effect, and then, for all intents and purposes, becomes a wendigo.

The possession and transformation lasts forever, unless the spirit ends it as a bonus action, the wendigo's heart is properly destroyed (see frozen heart), or the spirit is destroyed by an ancient ritual (see frozen heart). When the possession ends, the spirit reappears in an unoccupied space within 5 feet of the body. The target is immune to this spirit's Possession for 24 hours after succeeding on the saving throw or after the possession ends.