Slasher of Veils

The Slasher of Veils is a unbelievably powerful Daemon in Ultima Underworld I, most likely a Balron, whose banishment is the ultimate goal. Not much is known about it, but it seems pure evil. No mortal weapon and spell can destroy it, and it is so powerful, it could destroy all of Britannia.

Tyball and Garamon accidently summoned it in the Abyss. Trapping it just in time, they were confronted with the fact that they simply couldn't send it back. Erecting the Chamber of Virtues around it, they tried to find a way. But Tyball was corrupted by it, and murdered his brother in cold blood. The Slasher however, was trapped in the chamber, held by magic.

The Avatar found it there on the 8th level in Ultima Underworld I, its bonds nearly broken. Using the eight talismans of Sir Cabirus, the Slasher of Veils was sucked into a pocket dimension together with the Avatar. It tried to kill him/her. However, the Avatar escaped just in time, collapsing the only exit, trapping the Slasher inside forever.

Trivia

 * Despite being told that nothing can destroy it, casting Armageddon, a mortal spell, WILL destroy the Slasher of Veils. Killing it otherwise is really impossible.