Ad Astra Per Arcana
The Serpent (called Mok’slyk by the ancient Flan peoples) is an entity of godlike power, believed to be the personification of arcane magic.
The Serpent appears as a long, snakelike, semi-transparent entity. The Serpent’s head bears draconic features, and several dorsal ridges run along its back.
The Serpent is said to be a member of a group of unfathomably old entities known as the Ancient Brethren, which though similar to gods, are not exactly gods, though some beings honor them as such. The Lady of Pain, Asmodeus, and Jazirian are also sometimes said to belong, or to have once belonged to this group, and supposedly Vecna is a descendant of the Ancient Brethren. There may also be a connection between the Ancient Brethren and the draedens and baernoloths born before the multiverse began.
The Serpent is believed to have personally instructed Vecna in the ways of magic. Vecna’s mother, Mazzel, told her son that the Serpent gains its power by devouring the souls of those who honor it.
Other rumors include that the Serpent is a guise of Asmodeus, or that the Serpent doesn’t exist at all. Perhaps it is is only Vecna’s own madness and insight whispering back at him from within the darkness of his own one-eyed skull.