The Crack In The Sky, also known as The Crack was an in-game entity that spawned late in Season 4 (June 30th 2018, Blast Off Event) And disappeared after spawning The Cube (Aug 24, 2018, Crack Closure Event).
The Crack formed after the Blast Off event of Season 4. About a few hours after the event, a mysterious portal was on the lonely lodge sign and after a few days the portal, which turned into a rift had sucked up the sign into another world/dimension. This also happend with a few other signs/buildings like the Durr Burger head, which returned at the start of Season 5 and the Noms sign which also appeared back on the island near risky reels. After the rifts sucked stuff in, it began spewing things out. One day a rift was found near a mountain which we all know as "Viking Village", it confused everyone until it spewed out an Anchor that looked like it was from the viking times. After that a carrage that looked like it came from the 1800's spawned on a hill near Moisty Mire and a Skeleton of some sort spawned in the middle of Moisty Mire. During this the giant crack in the sky would expand, which is how all the stuff spawned in.
During season 5 the giant crack could be seen dripping sand from the middle part. During the summer skirmish the Rift which had shrunk had turned purple (likely due to being corrupted by The Storm) and was shooting purple lightning at a certain spot near Paradise Palms. It would shoot this lightning at the cacti until there was none left. After the last cacti was destroyed the lightning went full power and re-incarnated itself into a cube which we know as "Kevin".