Ultima, like her other primordial brethren, was born during the dawn of time before the creation of heaven and earth as the demonic counterparts to the Seven Angels of Origin. Her and the other primordials ruled Hell undisturbed for an eternity with Rouge as it's overlord. When Bleu and Vert were resurrected completely loyal to Rouge after their attempt to overthrow him, Ultima, Blanc, and Jaune agreed to never fight each other directly, instead preferring to outsmart each other in things like games and who could incarnate in the material world first.

Many, many, millenia later, after having countless failed incarnation attempts, Ultima was approached by her fellow Primordial Noir. Noir, now going by the name Diablo, "coerced" Ultima into serving his new master alongside him. Ultima went along with it, only intending to half-heartedly lend her strength for some years until she got bored, only to discover that Diablo's new master was indeed someone worth pledging her allegiance to.

(Source: Tensei Slime Wiki)