When the Ancient Spirits of Evil—behemoths enslaved by Mumm-Ra and Skeletor—attack Castle Grayskull, the crown jewel of Eternia, it’s up to Man-At-Arms, Panthro, Tygra, Teela, Cheetara, Wilykit, Wilykat, Snarf, Orko and a whole host of Masters to protect the Power of Grayskull!
The Achient Spirits of Evil have laid waste to most of King Randor's army and are posed to attack Castle Grayskull. With the help of the Thundercats, the Masters attempt one final all out assault on the Spirits, but they come up short, and the Spirits advance. Then Grayskull disappears and Jaga and The Sorceress appear to the Thundercats and Masters. The two had used their combined power to bring several powerful artifacts for the heroes, from Thundera, and they used the Golden discs to make the Spirits vulnerable for a limited time.
The Masters and Thundercats use this to their advantage and destroy the Spirits, but Skeletor funnels their power into himself and transformed himself to Mumm-ator