The Destroyer, the Lord of Darkness and Eater of Worlds, was a horned demon summoned by Morgaine to aid in her conquest of the world. He was a highly dangerous entity, and even Morgaine was hesitant to release him. He was bound in silver, to which he had a vulnerability. He was able to destroy matter with a green energy and could also teleport.
The Destroyer's origin is unknown, though Morgaine talks about summoning him from fire and implies that he came from Hell.
During the final battle for Excalibur, Morgaine attempted to threaten Ace and Shou Yuing with the Destroyer, claiming they would become his handmaidens. Though he wanted to break through the chalk barrier, Morgaine didn't want to release him, allowing him access to his full powers. Despite this weakness, he was still able to destroy the hotel, allowing Morgaine to escape with the sword and then teleporting away. The Destroyer let the Doctor and the Brigadier follow them, since Morgaine would need to release him. When the Doctor gained the upper hand, Morgaine was forced to unchain the Destroyer and escaped.
The Destroyer planned to devour Earth, though the Brigadier did apparently kill him by shooting him with silver bullets. The Destroyer exploded with a green fire, destroying Morgaine's castle and knocking out the Brigadier. (Battlefield, 1989) via