The Blessed Isles became the Shadow Isles after the Ruination , when Viego’s attempt to use the Waters of Life to save Isolde triggered the cataclysm that corrupted the land. Riot’s lore says this happened more than a thousand years ago in Runeterra’s history, so there isn’t a single exact real-world calendar year tied to it in the story.

What changed

  • The Blessed Isles were originally a revered, enlightened civilization centered on the Waters of Life.
  • Viego brought his army there in a doomed attempt to revive Isolde.
  • The ritual shattered the boundary between the material and spirit realms, creating the Black Mist and the cursed Shadow Isles.

In lore terms

  • The transformation is usually described as happening during the Ruination event.
  • That event is the origin point for the Shadow Isles as they exist now, with undead, spirits, and the Harrowing tied to the curse.

Simple answer

So, in League of Legends lore, the Blessed Isles turned into the Shadow Isles over a thousand years before the present-day setting , during Viego’s Ruination.