Clasceithir

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

Incorporeal Movement. The clasceithir 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.

Surprising Attacker. The clasceithir has advantage on all initiative rolls and Dexterity saving throws, and it has a +3 to armor class beyond its dexterity bonus.

Greater Etherealness. The clasceithir enjoys Blink as an always active spell. 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.

Scare Factor. The clasceithir has expertise on Stealth and Intimidation and has advantage on any efforts to Turn Undead or Destroy Undead.

Multiattack. The clasceithir makes three attacks, 2 Withering Touches, and then either a Horrifying Visage, Slime, or, if available, Possession.

Withering Touch. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 17 (4d6 + 3) necrotic damage.

Horrifying Visage. Each non-undead creature within 60 feet of the clasceithir 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 clasceithir'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.

Slime. Melee Weapon Attack: +9 to hit, reach 5 ft., one target plus any other targets within 5 ft. of initial target. Hit: 17 (2d6 + 7) necrotic damage. The clasceithir attacks a single target with Slime, but all other allies of the target within 5 ft. of the initial target are also subject to the initial attack roll. Each non-undead creature successfully slimed must succeed on a DC 10 Wisdom saving throw or be stunned for 1 minute while it contemplates and attempts to clear away the ectoplasm that covers them. A stunned target can repeat the saving throw at the end of each of its turns, ending the stunned condition on itself on a success. 

Possession (Recharge 5, 6). One humanoid that the clasceithir can see within 5 feet of it must succeed on a DC 13 Charisma saving throw or be possessed by the clasceithir; the clasceithir then disappears, and the target is incapacitated and loses control of its body. The clasceithir now controls the body but doesn't deprive the target of awareness. The clasceithir 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 clasceithir ends it as a bonus action, or the clasceithir is turned or forced out by an effect like the dispel evil and good spell. When the possession ends, the clasceithir reappears in an unoccupied space within 5 feet of the body. The target is immune to this clasceithir's Possession for 24 hours after succeeding on the saving throw or after the possession ends.

Cunning Action. The clasceithir can take a bonus action on each of its turns in combat. This action can be used only to take the Dash, Disengage, or Hide action.

Withering Response. The clasceithir can use Withering Touch as a reaction once per round.