The Destroyer

The Destroyer, also known as Lord of Darkness and Eater of Worlds, was a horned demon (TV: Battlefield) from outside of time and the universe. (COMIC: Cindy, Cleo and the Magic Sketchbook) The Seventh Doctor believed him to be "one of the 7,405,926 demons on the Talmudic table", more precisely a member of "what Ace would call the Top Ten". (PROSE: Battlefield)

Biography
After being released from the Circle of Transcendence, the Destroyer went to Othros Mu, where he destroyed a large city before being attacked by Sutekh and thrown back towards the Circle. This caused the flow of energy into the Circle to be blocked, preventing Sutekh from being sucked back towards the Circle. (COMIC: Old Girl)

The Destroyer was 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.

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 Seventh 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. (TV: Battlefield)

Other references
In the video game Happy Deathday, played by Izzy Sinclair on the Time-Space Visualiser, the Destroyer was among a host of "every single enemy" that the Doctor had ever defeated, who were assembled by the Beige Guardian and pitted against the Doctor's first eight incarnations. (COMIC: Happy Deathday)

Behind the scenes

 * Before scheduling changes were made, the Destroyer originally had a very different design done by Mike Tucker, who was reassigned to work on The Curse of Fenric instead of Battlefield. The new visual effects designer assigned to the story chose not to use this design, but it was retained by the BBC Visual Effects Department and later formed the basis for a monster in the Red Dwarf episode Terrorform.
 * According to Susan Moore and Stephen Mansfield, the Destroyer was initially intended to transform from a human to a demon by removing his chains, and his talons would burst through the skin of his hands. This was abandoned due to being too complex, and the Destroyer only appeared in his demon form in the story.
 * Moore and Mansfield also had to redesign the Destroyer's mask from the original maquette to add lips, so Marek Anton could deliver his lines while wearing it.