Shadow Vermin

"There has been much debate among the Horadrim regarding the shadow vermin. Some scholars believe that these wraithlike foes are formed from concentrated demonic energy, while others believe they are the souls of humans corrupted by Diablo. They glide through the darkness as easily as water, ready to smother their unwary prey."

- Deckard Cain

Shadow Vermin (a.k.a. Gloom Wraiths, Shade Stalkers, Terror Spawn, or just Shadows) are demons serving as 'horde' enemies in Diablo III.

Lore
"I have seen the shadow vermin in my dreams. They rushed in around me in a tide of inky black that reflected no light. Long arms and narrow faces rose from the darkness. They went up around my legs, then over my chest. I screamed in terror as they filled and blinded my eyes, but when I stopped fighting a calm washed over me. The darkness was pleasant. I felt whole as I floated through it."

- Adria

Shadow Vermin are wraithlike demons with long arms and narrow faces. They glide through the darkness as easily as water, ready to smother their unwary prey. However, their origins are a subject of debate. Some scholars believe that these wraithlike foes are formed from concentrated demonic energy, while others believe they are the souls of humans corrupted by Diablo. Adria wrote that Shadow Vermin are a concentration of Diablo's power; the fears of all mankind made manifest.

In-game
Shadow Vermin are the most numerous and weakest of all enemies encountered in Acts II, III, and IV. The apotheosis of their swarming rush is the battle with Iskatu at the beginning of Act IV. While they have no special abilities and attacks, they are quite fast, and their melee blows, while weak, are delivered at fast rate, and due to the appearance of these demons it may be challenging to see them in time (and even determine how many are you facing at the moment once encountered). They are encountered for the first time in the Realm of Shadow in Act II.

In the Shrouded Moors and Temple of the Firstborn, one may encounter identical, though slightly red-tinted, Blood Vermin. Their most notable appearance is during the Blood Statue / Ascension Ritual s and fight with Vidian.

Visually, they resemble pitch-black pools of smoking liquid that stick to the ground, with humanoid forms emerging when they attack.

Trivia

 * In form and function, they appear to the Destruction Spawn in Diablo Immortal. In-universe however, these appear to be separate types of demons.