https://www.gamespot.com/articles/the-fortnite-cube-melted-into-loot-lake-turning-it/1100-6461891/

The saga of the mysterious cube in Fortnite: Battle Royale has seemingly come to an end, just before Season 6 arrives. Following its debut earlier this season, the giant purple cube has traveled all the way to Loot Lake. Once there, it either sank or melt into the water, altering the makeup of the entire area.