Gloomstalker

Gloomstalkers are flying monstrosities native to the Shadowfell.

Appearance
"Black as night against the gray sky. From this distance, you make out no details in their bodies. It's almost like a moving shadow above you. Wherever light would hit to give you some sense of structure, it's just absence, void. It's just a flying shadow."

"It is billowing black shadow like smoke off of it. You can see glimpses of some sort of physical form in there, but it's so blurry, it's almost like it's phasing between physical matter and dream. You're not sure what it's made from, but you can see what looks to be an elongated neck, some sort of snapping jaw at the end, and two bright, greenish-yellow glowing beacons like eyes at the front of its face. Its wings don't have skin or flesh, it's just dark smoke, and as it flaps, it leaves these trails, these curling bits like black, black, black smoke. It's wyvern-sized. It's maybe 20 feet from wingtip to wingtip."

Behavior
The gloomstalkers were intelligent enough to coordinate an air patrol above Thar Amphala. They were able to communicate with one another and obey commands given via Dominate Monster.

One of the gloomstalkers encountered by Vox Machina showed evidence of being able to dispel an illusion of a little girl, produced by Scanlan. However, the gloomstalker was not intelligent enough to understand why the little girl disappeared when its anti-glamor cone crossed her.

Abilities
Gloomstalkers are able to fly at a speed of 80 feet per round and they have the ability to teleport short distances. Gloomstalkers are vulnerable to radiant damage.

Vox Machina encountered them in the Shadowfell, above the city of Thar Amphala.

While patrolling the skies above Thar Amphala, one of the gloomstalkers caught sight of Vox Machina as they were crawling under a wall into the city. It grabbed Keyleth, the last one to go through, and pulled her back. Scanlan quickly cast Dominate Monster and took control of it, telling it to release Keyleth and tell the other gloomstalkers that the hunt was off. It obeyed and, after a short discussion, the party gave the creature the name "Mr. Mistoffelees".

Vax'ildan and Scanlan later rode Mr. Mistoffelees up to scout out the ritual taking place atop Entropis.

Arkhan the Cruel took control of a gloomstalker using the Wreath of the Prism to cast Dominate Monster. This creature, dubbed "They" by the dragonborn, became his new mount before he and Vox Machina were attacked by Lord and Lady Briarwood and their own mount: an undead white dragon and previous mount of Arkhan.

The Mighty Nein encountered them in the Barbed Fields on their way towards Bazzoxan. The group inadvertently disturbed a group of gloomstalkers when they were eating an udaak they had just killed.

Trivia

 * Gloomstalkers are sexless.
 * Gloomstalkers share a name with a ranger subclass introduced in Xanathar's Guide to Everything, called the "Gloom Stalker".