Slasher of Veils

The Slasher of Veils is an unbelievably powerful Daemon in Ultima Underworld I—possibly a Balron—whose banishment is the ultimate goal. Not much is known about it, besides it seeming to be pure evil. No mortal weapon or spell can kill it, and its very presence threatens to destroy all of Britannia.

Tyball and Garamon accidentally summoned the Slasher in the Abyss. Trapping it just in time, they were confronted with the fact that they simply couldn't send it back. Situated in the Chamber of Virtues (the former Chamber of the Codex), they tried everything, but Tyball was corrupted by it and murdered his brother in cold blood. The Slasher, however, was trapped in the chamber, held there by magic.

The Avatar encountered it there again, on the eighth 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, whom it attempted to destroy. The Avatar managed to escape just in time however, collapsing the only exit and 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 through any other means is truly impossible.
 * In the original Bob White Plot of Ultima IX, the Slasher of Veils was planned to appear. As seen here in a preview of the game.