The Fearsome Foursome encounter Leon in Murder Mode

Leon was a Guardian Robot, built by Tharpod scientists Frumpus Croid and Nevo Binklemeyer like the rest of his. Originally built for usage in extracting the glowing shards of Terrawatt Forest (which were actually the remains of planet Toranux), it and other Guardians were quickly repurposed as war bots by Frumpus Croid following the Collectors assault on Uzo City. Leon and fellow Guardians were successful in protecting Frumpus Croid's base of operations in Terrawatt Forest secure, easily crushing Collector incursions in the region.

When the Fearsome Foursome needed to get to Croid, they activated Leon, and managed to get him out Murder Mode using a Cranial Mood Reshifter, causing him to ally with them. Qwark proceeded to name him Leon, and he aided them in getting to their destination - battling several other Guardian Bots along the way.