Lethe

Lethe is the last and most powerful of the Three, who conspire to keep the dead Praecor Loth in the dark about his own death.

Lethe was discovered by Praecor Loth in an arena fight and she rose quickly in the ranks. Although only holding the rank of a Captain, she nonetheless was the most deadly fighter in Loth's whole army, many thinking of her as a monster. That also helped to totally over-inflate her ego. So when Praecor Loth died, she, like Morphius and Lord Umbria, had become literally addicted to her king. She couldn't imagine existing without him, so the mage Lord Umbria turned all three of them into Liches and they held the soul of their dead king on the world, egoistically serving their need to have him around and denying everyone else their peace.

Many years passed and no one managed to challenge Lethe, until the Avatar managed to destroy Umbria and Morphius in Ultima Underworld II. Lethe by that time had gone totally mad. Her madness mixed with her ever inflated ego made her think she'd been even more important that she'd been (she even thought she and Loth had been lovers!), before attacking the Avatar.

The Avatar slew her and her essence was destroyed for all time. Ironically her egoism in the end prevented her from being with her king in the afterlife.

Trivia

 * Lethe seems to slowly fall apart, exactly like Morphius. Only Lord Umbria seems immune to it. Probably, the transformation into Liches doesn't work so well on non-mages.
 * Lethe's claim that she and Loth had been lovers is substantiated by Loth's wife Helena, who claims that her husbands "murdrous passion for wanton Lethe" actually helped contribute to their fall against the guardian.