The Overlord looks like a giant, green jellyfish with unnatural black eyes that resemble snail's, in which those eyes are similar with it's Jellion minions. It's internal organs (brain and what appear to be 2 hearts), which jellyfish lack, can be seen through it's transparent body. It has a huge mouth located on the lower part of his body.
It appeared that the Overlord's species are having a matriarchal system, as it serves as Alpha/Queen for the Jellions within the hive. The Jellions that it creates serve as drones/scouts, while Jellion Clones serve as some sort of soldier to defend it's lair.
Abilities and talents
The Overlord has ability to create Jellions by using DNA from a jellyfish that it consumes. It also has telepathic connection with the rest of the Jellions as had it killed, the rest of the Jellions would die along with it.
The Evil Alien Jellyfish Overlord looks like a giant, alien jellyfish and had the ability to eat regular jellyfish and create Jellions in their place. It then tells the newly made Jellions to eat everyone in Bikini Bottom, turn them into Jellion Clones, and take away the real copies. Near the end of the episode, the Overlord somehow ends up in the Krusty Krab (probability flew/swam over there) where it held everyone, even SpongeBob's spatula, captive in pods inside the freezer. When SpongeBob squirted mayonnaise on it, it melted away along with all the other clones. After he and the Jellion clones were melted, the remains were used for Relish Patties.
Where the Evil Alien Jellyfish Overlord comes from is currently unknown as it was first seen in a spooky cave at Jellyfish Fields.
It may have come from space since it is an alien.
It is also unknown how the Overlord ended up in the freezer of the Krusty Krab or why it went there. It could've swam there.
The Overlord may have stayed in the freezer because the alien knows there's mayonnaise, the weakness of it, will become frozen in the cold freezer.