Illuminus is a towering colossi that lives at Relic Lake where Katara needed to fight Hydrus. He is a very powerful semi-aquatic colossi who keeps Hydrus as a pet, probably the only colossi to do so. He is also one of JD's colossi on guard duty for the Hidden Lands, in his case it's the Relic Lake alongside Hydrus, making him and Hydrus one of three dynamic duos of colossi.
His first appearance was a little bit after Katara and Hydrus started fighting. He rises out of the lake slowly as he calls her back. He goes along with a usual colossi questions (why she's here, who she is, etc.). After that he sends out Hydrus once more. After Katara starts to win and to create a whirlpool, Illuminus sees her and tries smashing her but Katara already planned this and leaves an ice trap for him, leaving him with huge ice shards in his fist. Hydrus was launched at him, getting jabbed by Hydrus's dorsal fins and being killed as a result.