During the beginning of the episode after SpongeBob passes by a creepy cave, the Evil Alien Jellyfish Overlord eats a jellyfish and then makes a bunch more Jellions. One of the Jellions eats Patrick and makes a clone of him, thus Jellion Patrick is born. All the rest of the Jellions are then captured by Jellion Patrick (SpongeBob caught one for himself still thinking it's a regular jellyfish, and thought it was a new breed, because they didn't sting) and they both give out the Jellions to the citizens of Bikini Bottom. Afterwards, the Jellions managed to take over most of the citizens except SpongeBob and Sandy who both learn that the Jellion's weakness is mayonnaise. They then fought through the Jellion army using mayonnaise and eventually took down the Overlord. The Jellions were all destroyed, never to be seen again.
Jellions tend to like cold environments, and will make their nests in the cold. Jellions also seem to be weak to hot environments, shown after the Evil Alien Jellyfish Overlord dies. However, they will survive in normal temperatures, as they survive in Jellyfish Fields.
Jellions look like regular jellyfish, except they have big, cute, pitch black, shiny round eyes and mouths with sharp teeth in which they use to consume people and make Jellion Clones. When creating Jellion Clones, they generates tendril-like tube that used to secrete green biomass that quickly molts into the said Clones.
Being a mere clone of Jellyfish, they can't sting.
Abilities, Talents, and Relationships with Overlord and Jellion Clones
As creation and servant for Overlord, it appeared that Jellion taking role as both scout and drone like drone ants/termites does with their queens. This because while working on capturing non-Jellion lifeforms and creating the Jellion version of the said lifeform while taking away the original for the Overlord, they also scout over non-Jellion lifeform territories and stumbles upon Bikini Bottom that provided them more prey. While they appeared to be weak (noticably as they cannot sting), they counteract this via manipulation and trickery, as shown where it uses it's appearance for fooling SpongeBob. However, if necessary, they also willing to fight to the death with their enemies, though how they fight the threat is unknown (if any aside consuming them) as they immediately dispatched by SpongeBob and Sandy.
Primary Jellion has alarming ability to copy the captured victim's DNA and possibly their memories (while sedating them during the consuming process) that both used to create the Jellions Clones (which considered as Secondary Jellions). As result, while it's appearance mirroring the original except for the eye that remain same with other Jellions, Jellions clones would easily blend with non-Jellion lifeforms while either hunting for more potential victims or guarding the Hive/Overlord's lair.
The name "Jellion" may have come from the words "jellyfish" and "alien."
It is confirmed that even though they are an exact copy of a jellyfish, they cannot sting.
The Jellions are a parody of the plants from Invasion of the Body Snatchers.
They have a weakness to mayonnaise.
They have less spots than any other jellyfish in all forms.
It's unknown whether Jellion's appearance only taking form of jellyfish or can also taking other lifeform instead, though this possible as any lifeform's DNA that consumed by Overlord can be used to create Jellions.