Ethereal Monster

The Ethereal Monster is an entity which exists to guard the Guardian's Tetrahedron Generator in Ultima VII.

Description
A gargantuan red-skinned creature, the Ethereal Monster existed within the the confines of the Tetrahedron, where it would attack the Avatar once they breached its defenses using the Ethereal Ring. Far stronger than most enemies on the physical plane, this beast could easily slay even a strong and well prepared combatant in a matter of moments. Nevertheless, the Avatar managed to best it, and was able thereafter to destroy the blackrock construct which had disrupted the ether of Britannia.

Trivia

 * In the SNES Port of Ultima VII, the Ethereal Monster exists in a chamber before the Tetrahedron Generator. The Avatar can only slay it, however, while wearing the Ethereal Ring, which aligns the hero and their weapon with the ethereal plane on which the creature exists.
 * The best option is to use the Black Sword, as its death-command can kill the beast in one strike. If that option is not available, a Glass Sword is the best way, combine with the spell Mass Might.