The CUBE Event was a Live Event that that took place on August 24th, 2018 in Season 5.
The day before, in Patch v5.30, the rift in the sky shrunk to a very small size and turned purple. The day after, it began hitting a circle of cacti in coordinate H9 with lightning strikes. It destroyed each cacti one by one, until when it destroyed the last one, the lightning strike was put out for much longer, and afterwards the CUBE appeared there. The rift then shrunk for the final time, disappearing from the sky.
The CUBE then began to move around the map, planting runes in specific locations for the lead-up to Season 6.