The Cluster is the final product of a numerous amount of Cluster Gems, and the supposed "doomsday device" to destroy Earth. While not an antagonist, it is the driving force for the Crystal Gems throughout the first half of Season 5.
As a whole being, the Cluster is a gigantic orb of epic proportions, the gemstone core itself dwarfing any creature on Earth. It's unclear as to just how huge it is, but it's implied that emerging itself will be enough to destroy the entire planet Earth. Despite its giant size, it does present a form of a extremely small, simplified, transparent, humanoid figure no bigger than Peridot, mainly as a representation of itself.
It's unclear to exactly how it behaves, if it does indeed have a behavior, though during its encounter with Peridot, it was able to recognize that Peridot was hurting, though didn't comply to stop forming, as it wanted to be whole. It is pointed out by Steven that it felt very lonely, but then wanted to stay when realizing it had hundreds of friends inside it already.
While unknown of what it can do, it is shown to be able to bubble itself in small intervales, even when inactive.
The Cluster was first mentioned by Yellow Diamond in an encoded message brought to the Crystal Gems, described as a weapon of sorts. Peridot described it further as a being composed of millions of Gem shards, and the result being catestrophic once it does emerge and takes form.
The Cluster began to take on its form during the series's middle two-parter, and while it was trying to, discovered Peridot on its surface. It appeared as a small humanoid figure to her, but still refused to comply with her despite the damage it would cause. When Steven Universe ended up inside it, it stated that it wanted to be whole, telling him that it was lonely. With some convincing, it wanted to change its time, but found it couldn't stop forming, pleading for help. With the aid of Steven, Peridot, and Pearl, the Cluster bubbled itself. It is currently bubbled within the Earth, now content with itself.
While not making a physical reappearance, White Diamond threatened to release the Cluster a number of times during Season 6, one time for Steven to comply to her dog, and second when she demands Peridot to release the Cluster directly.