https://www.gamespot.com/articles/fortnite-a-giant-mysterious-cube-rolls-around-the-/1100-6461395/

It's been quite the day in Fortnite. After weathering a massive lightning storm, players discovered a mysterious glowing cube in the desert. No one knows exactly what it's for yet. [Update: Its purpose remains a mystery, but the cube has begun moving around the map at regular intervals. We'll report back with any further developments.]

Earlier today, purple lightning began striking the desert. Players quickly observed that the bolts were coursing from the rift in the sky, and that some were striking and destroying cacti. The lightning strikes started coming faster and faster, until they suddenly just stopped. In the aftermath, players discovered a massive, seemingly interdimensional cube on the edge of the desert. The rift is now gone as well.