The Dark King, AKA The Dark, is the true antagonist of the Ultima series.
According to legend, he was a ruthless king whose tyranny impressed a dark god, who in turned gave him some of his powers. The Dark King proved to be much more eviler then the dark god, so the evil being devoted what was left of it's power to him. Eventually a warrior gather the Ultima weapons, and defeated him. That warrior is supposed to be Nick's ancestor.
But this... this... is all just a legend. Hardly anyone knows the truth.
His origins will be revealed in the story, "Ultima: Origins of Darkness".
The Org plans to resurrect the Dark King, using 101 numericals.
Until then an Avatar of the Dark King attacked Nick at the begining of the Crossing of Universes.