The Evil Alien Jellyfish Overlord is the main antagonist of the Spongebob Squarepants episode "Planet Of The Jellyfish".
Much like the Pod People that it based on, Overlord are member of alien species called Jellions that replace the native species of a planet that it invade by spawning emotionless replicas.
During the beginning of the episode, after Spongebob passes by a creepy cave, the ruler, whom notice their presence eats a jellyfish, assimilating it and proceed to makes a bunch more Jellions. It knew that SpongeBob and Patrick are hunting jellyfishes, and send some of his Jellion Jellyfishes after them.
One of the member of squadron of Jellion Jellyfishes eats Patrick and makes a clone of him, and thus Jellion Patrick is born. Once the Jellion cloning process finished, it had Jellion Patrick to do it's work while carrying the unconscious real Patrick to Overlord off-screen. Jellion Patrick, whom proceed to manipulate SpongeBob, had the rest of the sent Jellion Jellyfishes squadron pretend to be captured by it. The manipulation tactic was succesfull as SpongeBob, whom also caught one for himself was not suspicious with one of it's members, and still thinking it's a new breed because they didn't sting. Jellion Patrick and they both give out the Jellion Jellyfishes to the citizens of Bikini Bottom, unaware that they mere duplicate.
Rising to Power
As this happens, Overlord, with unknown means of transportation, arrives on Krusty Krab and proceed to building a nest within the freezer room by the time the Jellions managed to take over most of the citizens except Spongebob (whom realized that invasion has began and discovering the Hive) and Sandy Cheeks (whom notices the Jellions' presence due to their unnatural eye shape in spite of imitating Bikini Bottomites). They both learn that the Jellion's weakness is mayonnaise due to them never wanted nor seen eating one. This proved to be correct as when both of them surrounded by Jellions on the Krusty Krab, SpongeBob shot one of the jellion with a mayonnaise which killed one of them and reduce their body into green goo.
Final Battle and Defeat
By the time they killed presenting Jellions, their original counterparts, which cocooned within the nest, awakens (implies that the Jellion clones had telephatic connection with the cocoon that restrain the original and had they killed, the cocoon would rots and free the said original). Patrick and the kidnapees are cornered by Overlord and the Jellion Jellyfishes until SpongeBob and Sandy bursts in. The brief battle occurs, where SpongeBob blasts all of it's Jellion Jellyfishes with mayonnaise and tries to spray one to Overlord. But due to all of them are in freezer room, the sprayed mayonnaise that intended for Overlord freezes on the midair and land on it's head in form that resembles a marsh mellow. However, Sandy quickly set the temperature into warm where the mayonnaise eventually melt, killed the Overlord and it's fellow Jellion dies due to their telephatic connection.
Overlord looks like a giant, green jellyfish with unnatural black eyes that resembles snail's, in which those eyes are similar with it's Jellion minions. It's internal organs (brain and what appeared to be 2 hearts), which jellyfishes lacks, can be seen through it's transparent body. Overlord's mouth, which is huge, are located on his body.
Powers and Additional Information
Since Overlord only appeared in one episode, not much additional information that we could learn about it of it's race. So far, the shown abilities was it can consume a nearby living creature in order to creates a number of Jellion version of the said creatures' species like it did to an unfortunate jellyfish that stumbled upon it. These newly created Jellions became the Primary Jellions, which would later engulf any non-jellion lifeforms that they encounter, creating their Jellion duplicates, and then bring the original to Overlord that create it. Jellions that created by Primary Jellions are Secondary Jellions, that would assist the Primary Jellions as well as taking over live that the original had in their normal days.
Being a race that portrayed as having a matriarchal system, Jellion Overlord serves as Alpha/Queen for the Jellions within the Hive. It also has telepathic connection with the rest of the Jellion Hive as had it killed, the rest of the jellions would die along with it.