Shaco and the Ruined King
Theory: Shaco used to be the Ruined King's court jester From Shaco's lore: "Crafted long ago as a plaything for a lonely prince, the enchanted marionette Shaco now delights in murder and mayhem. Corrupted by dark magic and the loss of his beloved charge, the once-kind puppet finds pleasure only in the misery of the poor souls he torments." The cause of Shaco's corruption is dark magic and the loss of his friend, the prince. The lore mentions that he was crafted for a prince, but it does not specify wether this prince was still a prince at the time of his death or already a king. We know from Kalista's and Hecarim's lore that the Ruined King committed suicide with his wife after she was resurrected and begged for death. He also cast a spell to bind their souls for all eternity, but the many magical artifacts stored on the island massively amplified it and created a black mist that swept across the island, killing everyone and giving birth to what we now know as the Shadow Isles. Since Shaco was created because of the prince's loneliness, it's likely that he was his only friend and would follow him on expeditions like that of the Ruined King to the magical isles to resurrect his wife. The suicide of the king and the black mist would fit into Shaco's lore as "the loss of his beloved charge" and the "dark magic" that corrupted him.