Medium undead, chaotic evil
Armor Class 16
Hit Points 216 (27d8)
Speed 0 ft., fly 40 ft. (hover)
STR 8 (-2) DEX 16 (+3) CON 10 (+0) INT 10 (+0) WIS 11 (+0) CHA 13 (+1)
Skills Medicine +4, Nature +4, Perception +4, Survival +4
Damage Resistances acid, fire, lightning, thunder; bludgeoning, piercing and slashing from nonmagical attacks
Damage Immunities cold, necrotic, poison
Condition Immunities charmed, exhaustion, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained
Senses darkvision 60 ft., passive Perception
Languages Abyssal, Gnoll
Challenge Rating 15 (13,000 XP)
Ethereal Sight. The shoosuva can see 60 feet into the Ethereal Plane when it is on the Material Plane, and vice versa.
Illumination. The shoosuva sheds bright light in a 10-foot radius and dim light for an additional 10 feet.
Incorporeal Movement. The shoosuva 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.
Rampage. When the shoosuva reduces a creature to 0 hit points with a melee attack on its turn, the shoosuva can take a bonus action to move up to half its speed and make a bite attack.
Turn Immunity. The shoosuva is immune to effects that turn undead.

Multiattack. The shoosuva makes two attacks: one with its bite and two with its claws.
Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 17 (3d8 + 3) piercing damage. If the target is a creature other than an undead, gnoll or hyena, it must succeed on a DC 16 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) piercing damage.
Etherealness. The shoosuva 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 that is not a gnoll or hyena within 60 feet of the ghost that can see it must succeed on a DC 16 Wisdom saving throw or be frightened for 1 minute. 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 shoosuva’s Horrifying Visage for the next 24 hours.

Spoiler: Shoosuva Lore
The shoosuva are spoke about in whispers in regions ravaged by Yeenoghu's followers, the phantoms being some of the most feared beings in Yeenoghu's arsenal.
Shoosuva appear as large, emaciated humanoid hyenas that walk around on all fours. Their spectral bodies glow with yellow light, and their eyes shine a bright white.
Summoned Aid Shoosuva are rarely found on the mortal plane by choice. Rather, most shoosuva are summoned via a special amulet. Knowledge of this amulet's creation is known only among the highest ranking members of Yeenoghu's cult, likely told by the demon lord himself. Using this amulet, the cultist can summon shoosuva in battle to aid them.
A Fanatic's Fate The origins of the shoosuva are shrouded in mystery. Though a couple of theories have sprouted up, the most commonly believed one is that the shoosuva are in fact the spirits of the most powerful and devoted of Yeenoghu's followers. It is said that when they die, Yeenoghu uses his power to draw their souls to the Abyss, transforming them into shoosuva.

Spoiler: Personal Notes
The shoosuva are not completely my idea, but rather a 1st edition monster apparently created by Gary Gygax himself (that's what I heard at least). I do hope I got the calculations for the CR correctly, for I'm new to the whole monster creation business and could easily have gotten something incorrect. Anyways, hope this is at least somewhat alright