Superboy and the Legion of Super-Heroes #229

While some of the more powerful Legionnaires travel to space to search for Deregon and the Dark Circle, the three founding Legion of Super-Hero members, Saturn Girl, Lightning Lad and Cosmic Boy, learn that Deregon is actually on Earth. Knowing there isn't a lot of time to get Deregon, they go after him and find him, with a fight ensuing. In the confusion of battle, Deregon falls into a pit of chemical wastes and is trapped, a fitting irony since he killed Chemical King of the Legionnaires.