What is an Anti-Prime? Some answers can be found in this excerpt from Landmine's essay “The Lost Primes: Evolutions of an Institution Through the Ages”. History is written by the winners. This is a common concept that assuredly explains the reason why so little is known about the Anti-Primes.
Thanks to enduring fragments of information handed down through oral traditions, it can be safely assessed that at recurring points of our history, many Primes existed simultaneously; and more or less, they waged open conflict between themselves over which was the genuine Prime. The victor would acknowledge his triumph by erasing the opponents not only from existence (banning them elsewhere, or killing them), but from history as well.
Through indirect means, however, a small amount of information has become available (oral tradition, as already said, but also artistic works such as frescos or bas-reliefs that survived the purge), allowing a reconstruction of basic facts.
For instance, by now it is ascertained that during the Age of Nova Prime, a Guardian Prime arose to challenge his authority, apparently in conjunction with the Abyss Resurgence. Little is known about him, his purposes, his supporters or his possible relation with the Source. Was he supported by a splinter group of Ancients? Did he receive an Anti-Matrix? These, as many other questions, remain without an answer.