Darth Noctyss was a Sith Lord searching for immortality on Exegol in the story “A Life Immortal” from the book Star Wars: Dark Legends by George Mann.
From “A Life Immortal”
Celebrated and abhorred in equal measure–for even as a young woman her power set her apart from others–Darth Noctyss had long carved a path of red destruction through the Inner Rim. Worlds fell to her sickle-bladed lightsaber, and the ruins of the civilization of Ivis stand as testament to her hatred.
I mean, she sounds fabulous.
Darth Noctyss has not appeared in any other Star Wars stories … yet.