The version appearing in Fate/Grand Order is from a timeline where he never returned Excalibur to the Lady of the Lake, resulting in Artoria Pendragon being unable to die. He ended up waiting in the Garden of Avalon with Merlin for 1200 years until the events of the Camelot singularity, where he was finally able to defeat Artoria, now having been taken over by the holy power of Rhongomyniad and becoming the Lion King.
