It was the leader of the alien jellyfish known as Jelliens and almost succeeded in turning everyone in Bikini Bottom into jelliens until Sandy and SpongeBob showed up. He is first seen in a cave in Jellyfish Fields watching SpongeBob and Patrick.
The Jellien Leader looks like a giant, light green jellyfish with unnatural black eyes that resemble those of snails, in which the eyes are similar with its Jellien minions. Its internal organs (brain and what appear to be 2 hearts), which jellyfish lack, can be seen through its transparent body. It has a huge mouth located on the lower part of his body and 15 squirmy legs.
It appeared that the jelliens' species are having a matriarchal system, as it serves as Alpha/Queen for the Jelliens within the hive. The Jelliens that it creates serve as drones/scouts, while Jellien Clones serve as some sort of soldier to defend its lair.
Abilities and talents
The Jellien Leader has the ability to create Jelliens by using DNA from a jellyfish that it consumes. It also has telepathic connection with the rest of the Jelliens as had it killed, the rest of the Jelliens would die along with it.
Role in episode
The Jellien Leader looks like a giant, alien jellyfish and had the ability to eat regular jellyfish and create Jelliens in their place. It then tells the newly made Jelliens to eat everyone in Bikini Bottom, turn them into Jellien Clones, and take away the real copies. Near the end of the episode, the Overlord somehow ends up in the Krusty Krab, where it holds everyone, even SpongeBob's spatula, captive in pods inside the freezer. When SpongeBob squirts mayonnaise on it, it melts away along with all the other clones. After he and the Jellien clones were melted, the remains were used for Relish Patties.
The Jellien Leader manages to clone almost all of the main characters except for SpongeBob, Sandy, and Plankton (who wasn't seen anywhere during the episode).