Just a new champ idea and a bit of curiosity

Hey riot I've just learned about the blessed isles that were standing before the shadow isle while taking a look a the map of Runeterra, and I thought there could be some great materials for an ancient character with some connections with Ryze, who probably lived long enough to know about this secluded country. Also, the man with a staff on the picture of the celestial vaults in ruins, doesn't seems like someone that used to lived there, as if the place has been abandoned a while ago...

Just out of curiosity, did the white mist from the blessed isles gradually changed to the black mist, or was it a sudden change?