So, The Forgotten Evil kind of sums it up.
He's really old, so much so that he's been forgotten. Literally, there's no one around that remembers him.
Oh, and he's evil, really evil.
The Forgotten Evil will not be seen until the final story, The Last Queen. Until then, he is a shadow and a mystery. Any bad guys that come along in the meantime work in service of this guy. He's the reason they do what they do. The Forgotten Evil is nameless and shapeless. Maybe at one time, he was a person and there will be more on that in Book IV: The First Queen.
For now, he is the man behind the curtain, serving mostly as a plot device.
Being such an absentee master, he still manages to be quite a motivator. In the land of Empyrean where peace has reigned for hundreds of years, the Forgotten Evil has managed to muster quite an army: the snail-like General Gusk, dwarf Lord Vanril, Sasha the gypsy witch, carnival ringleader Raymond Shaydaway, Spynum Begg, Emperor Am-Li. Not to mention the hordes of underlings willing to do mean things: werewolves, Rockhorns, dwarves, half-dino warriors, black cloud monsters, or kappas.
The question is, who will win in the end? The Forgotten Evil or the queens?