Guardian

The Guardians are titanically colossal, very ancient gems of unknown gem type. Tougher, thicker and denser and moreso more powerful then even the Diamonds, the Guardians were an faction that destroys planetary bodies if it saw it as unfit for Gem colonization. During the Rebellion, the Guardians attempted to destroy Earth, but their efforts was thwarted by King Arthur, being sent into hyperspace by Excalibur.

In truth, the Guardians are the result of an perfected fusions of trillions of Gem shards, being the combining will and physical form of many thoughts. It's also said they're also the planets themselves, absorbing any planet and making it become them. Clusters they were originally were aeons ago, the cluster evolved in a way it drifted from the intended path, all the thoughts and will collectively becoming an single entity - a fusion that'll fused itself with the planet, and take form as an humanoid entity with the power over matter and energy.