He was in a mind control spell by Planktonamor. He attacked the village and kindnapped Princess Pearl. SpongBob, Squidly, and Patrick were all sent to rescue Krab's daughter. After the Dark Knight battle was over, the Dragon Jellyfish returns back to the wizard's palace and the letter shows to Krabs that he has to give him his kingdom or Pearl will suffer in boiling lava. The Dragon Jellyfish sees a Krabby Patty and takes it, then falls in love once he has eaten it. SpongeBob and Patrick have won and Planktonamor was zapped by the Dragon Jellyfish. He was seen again where King Krabs was making Krabby Patties and Dragon Jellyfish eats another patty.
The Dragon Jellyfish is a gigantic jellyfish with purple spikes on his head, long tentacles, the same jellyfish looks with red polka dots, and he has a dragon tail.