Nyx has pale red skin, overlarge black eyes and vents along her hairline, vents were also present on her neck and back as well as down the backs of her arms. She also has a small Cauldron emblem tattooed on her face like a beauty mark.
Nyx has the ability to release poisonous gas from the vents on her body which she forms into solid shapes that move at her will. If she chooses to release it or you break the shape it releases the gas. At best the gas will knock a person out, at worst it can cause permanent brain damage and organ failure. She also seems limited in how much gas she can produce. Presumably she has some form of Manton Effect that protects her from the effect of her power but this is unconfirmed.
A young instance of her was adroit enough with her gas to create illusions inside of illusions. The original Nyx was likely more skilled and may have had a stronger connection to her passenger. Another could overlay disguises on others, with the illusions mimicking what the covered individual was doing.
Nyx, and her twin sister Nix both took Cauldron vials, presumably as part of tests with capes with identical DNA, like Fenja and Menja. Only Nix had viable powers, while Nyx became a "monstrous cape". It appears that Nyx was released into the world as another Case 53, while Nix was allowed to continue her life as normal.
↑And in the midst of the crowd, there were the enemies, simply standing and observing. Nyxes, Psychosomas and Night Hags. The Nyx were women with pale red skin and black eyes, fog bleeding out of the vents at their arms and backs. - Excerpt from Sting 26.5
↑ 2.02.1Jack’s expression became a frown. Then he dissipated.
It was only a teenager, trapped inside. He was in the middle of asking a question. “-you let me out?”
“Holy fuck. I almost burned him,” Crucible said.
The boy pounded one hand on the forcefield. “Please!”
“I’ll let him out,” Crucible said.
I hesitated, holding up a hand.
No. Not enough grounding to say for sure. I let my hand drop.
I was about to give the go-ahead, but Tattletale’s voice came over the comm. “That’s Nyx you’re looking at. Her range is too short, she’d have to be in the building, and she’s too distinctive looking to pass in a crowd.”
I stared at the teenage boy. I’d almost said he could leave.
“Last chance, Nyx,” I told the ‘boy’. “Last words? Share a juicy tidbit?”
The ‘boy’ faded away. An illusion in an illusion. It was only a woman with pale red skin, overlarge black eyes and vents along her hairline, the back of her neck and down the backs of her arms. A fog seeped out from the holes. A small Cauldron emblem was tattooed on her face like a beauty mark. - Excerpt from Sting 26.3
↑“Nyx illusions,” Tecton said, “He doesn’t know we’re aware of who he brought, so he’s set them up to stall us.”
Nyx. Her gas is concentrated into solid shapes that move at her will. Break that shape and it becomes a cloud of poisonous gas.
“Not that easy,” Weaver said. “Maybe he knows we know, and it’s a double-bluff.”
I don’t like the illusory building faces. Too much poisonous smoke was needed to make that sort of thing, it had to be multiple Nyxes working in concert. They’d be close, probably. - Excerpt from Interlude 26a
↑Though it’s somewhat justified in this instance as they don’t retain their full memories – only cobbled-together personalities and histories. - Comment by Wildbow in Sting 26.3
↑The entity observed the ongoing conflict. No less than five seconds after it had been trapped, two figures had emerged from a doorway between worlds. The entity could see the paths forming, trace them back to the source. Another world, a living world without a shard occupying it.
They engaged the eight with their own perception abilities, intervening to assist a group of others. As a pair, they opened fire with guns, then waded into hand to hand combat.
↑ 6.06.1A Hatchet Face, Breed, Cherish and King made their way towards the entrance of the alley. Golem created hands to block their path. [...] They tensed, but the King looked over his shoulder at the Cherish, and when he looked up again, he was smiling. [...] The illusions collectively shattered as he squashed the head of the ‘Cherish’ against the wall. Nyx.