Eigeron's Ghost

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

The ghost 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.

If it is destroyed, the ghost re-forms after 24 hours. To truly destroy the ghost, characters must lay Eigeron's spirit to rest by killing Blagothkus.

Melee Weapon Attack +5 to hit, reach 5 ft., one target. 17 (4d6 + 3) Necrotic damage.

The ghost 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.

One humanoid or giant that the ghost can see within 5 feet of it must succeed on a DC 13 Charisma saving throw or be possessed by the ghost; the ghost then disappears, and the target is Incapacitated and loses control of its body. The ghost now controls the body but doesn't deprive the target of awareness. The ghost can't be targeted by any attack, spell, or other effect, except ones that turn undead, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunity to being Charmed and Frightened. It otherwise uses the possessed target's statistics, but doesn't gain access to the target's knowledge, class features, or proficiencies.
The possession lasts until the body drops to 0 hit points, the ghost ends it as a bonus action, or the ghost is turned or forced out by an effect like the Dispel Evil And Good spell. When the possession ends, the ghost reappears in an unoccupied space within 5 feet of the body. The target is immune to this ghost's Possession for 24 hours after succeeding on the saving throw or after the possession ends. If a creature possessed by the ghost is forcibly removed from the Eye of Annam, the ghost is expelled from its host and re-forms in the middle of this room.