Thresh, the sadistic Chain Warden of League of Legends, seems to be behind every malevolence in the universe's recent history, including the corruption of Blessed Isles into Shadow Isles.
Those who followed League of Legends lore in recent months are aware of The Ruined King's involvement in the creation of the Shadow Isles but here is a quick recap just in case.
Viego's wife got mortally wounded during an assassination attempt, his general Kalista sought cure and found Blessed Isles with mystical powers but bad things would happen should someone try to use their power in order to resurrect someone. Viego the Bratty King didn't care and he still try to bring back his wife and therefore ruined everything for everyone.
One bit of lore that was missing in that puzzle has been revealed on April 21, 2021. Namely, Thresh used to be a warden on the Blessed Isles but he apparently hated the overall happy place. He knew well that attempting a resurrection would corrupt the Isles but he still led Viego to the Waters of Life where the latter submerged Isolde.
The resulting disaster tore everyone's souls and ruined the land, making it known as Shadow Isles in the future. All of this is hinted to have been a plot to prolong Thresh's cruelty into undeath where he, as you may know by now, captured Senna and was a total douche to both her and her husband.
There is no telling what else he did during the long period of time the Isles were corrupted but we are getting a distinct feeling Riot are building him up as one of the antagonists and probably bosses of the upcoming League of Legends MMO.