We actually don't know either. We only know that a Monster of Alaya exists and the Monster of Gaia is Primate Murder, so the Monster of Alaya should be something equivalent to that, but on the side of humanity.
After 15 years of never being mentioned again, FGO's KnK has a special scene where Void Shiki randomly shows up and fights some 6 million HP ghosts. They are called the final form of ghosts who resented people too much and went crazy over death, and Void Shiki explains that they are bootlegs versions of the Monster of Alaya.
This is literally all the info we have at this point for a concept that was first presented in Kagetsu. Anyways, going by the ghosts' description, it looks like the Monster of Alaya is the unofficial Grand Avenger, so 3 out of my 4 candidates for that super versions of Angra, Dantes and Erice. The 4th candidate is Void herself due some assorted circumstancial evidence (the implications of her having Independent Manisfetation, Shiki's "Class" in /EXTRA being Monster, and how Fou got a big reaction to the ghosts so his Alaya counterpart suddenly turning Void would be an equivalent reaction).