Evil Alien Jellyfish Overlord

Evil Alien Jellyfish Overlord was an alien and jellyfish-like monster who was the species progenitor of the Jelliens. It kidnapped Patrick, Squidward, Mr. Krabs, Gary, and everyone in Bikini Bottom, except SpongeBob and Sandy and possibly a few others. It was the leader of the alien jellyfish known as Jelliens and almost succeeded in taking over Bikini Bottom until Sandy and SpongeBob showed up.

Biography
The Overlord began its invasion of the town of Bikini Bottom by swallowing a jellyfish and creating an exact copy of it (save for a pair of big black eyes), it then instructed it to capture Patrick and began making more. SpongeBob unwittingly assisted the clone of Patrick in spreading the Jelliens to everyone in Bikini Bottom, but eventually became aware of their plans after finding the leader in the freezer of the Krusty Krab, as well his friends encased in green pods. Though with Sandy's help, who became aware of the Jelliens before SpongeBob, he figured that mayonnaise was the Jelliens' weakness and destroyed them.

Weakness
The Evil Alien Jellyfish Overlord's weakness is mayonnaise. SpongeBob managed to find this out after the Jellien Clones in the Krusty Krab only ordered Krabby Patties without mayonnaise. Even one little squirt of the condiment will completely destroy any Jellien and Jellien Clone as SpongeBob and Sandy fought through the Krusty Krab using a jug of mayonnaise. The Overlord was eventually defeated after Sandy turned up the thermostat causing a block of mayonnaise (which SpongeBob threw at it but it froze and the block landed on its head) to melt on it. All of the Jelliens and Jellien Clones were killed and Bikini Bottom was saved.