Keldor, more commonly known as Skeletor is the primary antagonist of the Masters of the Universe franchise. He is a character appearing in the Turtles of Grayskull (mini-comics) and Turtles of Grayskull (comics).
Turtles of Grayskull[]
Through a mishap with a paradimensional tear, the Ninja Turtles are transported to Eternia, but so are their enemies Krang and Shredder. The Turtles arrive in time to assist Man-At-Arms and Teela to destroy a horde of Mousers and capture Mouse-Jaw. After sharing stories, Man-At-Arms is certain that Mouse-Jaw is proof that Krang has joined forces with Skeletor. On Snake Mountain, Skeletor compliments Krang on his plan and it is revealed that they have mutated He-Man By The Power of PIZZA!
When the villainous Beast Man shows up at Palace of Eternos, he is quickly captured. He says that Skeletor summoned an emerging power to Eternia, which turned out to be a bridge to another universe. This dropped Krang and Shredder in the middle of the Swamp of Gangoon. Skeletor struck up a partnership with Shredder and Krang to utilize mutagen to conquer first Eternia, then Shredder and Krang's world. Beast Man tells them that he witnessed Ram Man, Moss Man, and even He-Man himself captured and subjugated to the mutagen. Claiming that he came to the palace to seek help because he didn't want to be mutated, Beast Man agrees to lead a team to infiltrate Snake Mountain. When they arrive, they find that they have walked into a trap set by Skeletor. Belly of the Beast (Man)
Skeletor is joined by Krang and Shredder in leading a force of evil towards Castle Grayskull in an attempt to infiltrate and take over the power inside. Turtles of Grayskull issue 1