The Mirror Wraith is a demon wich hides in mirrors. He was summoned by the Ancient Spirits of Evil to help Mumm-Ra. Mumm-Ra disguised the Wraith as a child's doll and gave the doll to Leah, thus getting the Wraith inside Cats' Lair.
The Mirror Wraith is eventually defeated by Leah and Snarf when the pair use fire-extinguishers to cover all of the mirrors in Cat's Lair, trapping the creature. Upon being transported back to Mumm-Ra's pyramid, he had the Ancient Spirits of Evil deal with Mirror Wraith. They end up teleporting Mirror Wraith away.