Renekton was a war captain who contributed to Ancient Shurima's conquest and expansion efforts by fighting the wars his brother Nasus planned.
Renekton's self-sacrifice to help his brother survive by turning Ascended earned him the right as well. Over time, his newfound form awakened his inner fury, which would be later harvested by Xerath to drive him mad and turn him against Nasus.
Renekton and Nasus were driven away from the Shuriman capital thanks to Xerath releasing Brand from his sarcophagus, buying the Magus time to steal Ascension from Azir.
Renekton threw himself and Xerath into the Tomb of the Emperors and asked Nasus to seal it, which he reluctantly did. Millennia later, Sivir and Cassiopeia opened the tomb and freed them.
Renekton's insanity is the byproduct of Xerath's words and of the centuries he spent with him in utter darkness. The Magus brainwashed him into believing it's all Nasus' fault, and will hunt down his brother while butchering everything and everyone standing in his way.