Gloomstalker

"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."

- Dungeon Master of Exandria

A gloomstalker is a terrifying winged predator that resembles a wyvern and is composed of twisting shadows with glowing eyes and dagger-like teeth. Hailing from the cursed lands of the Shadowfell, these nightmarish creatures hunt the barrens of that desolate plane, freezing prey in their tracks with their woeful shrieks before pulling them into the skies with their talons to be ripped apart and devoured.
A gloomstalker’s proclivity for darkness makes it extremely dangerous at night, or in heavily shaded places where it can blend in with their shadowed surroundings. More dangerous than the lone hunter is a pack of Gloomstalkers - small packs of three to six are common, while larger packs of ten or more, called frenzies, are often harbingers of a greater evil yet to come.
In some worlds, gloomstalkers were employed by arcanists or the followers of the fallen Gods as mounts in calamitous battles. After being brought to the prime material planes for this purpose, the creatures remained and littered the darker areas of the landscape - wastelands, swamps, and the Underdark.