Last night, not long after I put my kids to bed, I made some coffee and turned on my PS4 to play Fortnite. It wouldn’t have made the best spectator experience: I sat perched on an in-game cliff for several minutes, barely moving. I wasn’t playing stealthily, trying to avoid detection from other players. I wasn’t even armed. Instead, I was staring intently at a giant purple cube, hoping it would move. Five minutes into the match — in which I somehow remained unharmed — my patience was rewarded. The rear end of the cube rose into the air slightly, hovered for a few moments, and then the big box rolled forward before settling once again into a motionless stance.

This is what Fortnite has done to me. I’m spending my time obsessing over a mysterious cube.

Everything started last week. Fortnite’s ever-evolving island had been relatively quiet for some time, but following a big update, things were changing. There was a strange temple dedicated to tomatoes, and new portable rifts that let players create miniature portals. But more bizarre was a series of lightning strikes that seemed to happen at regular intervals. Fans thought the strikes were some kind of countdown, and they turned out to be right.

Eventually, the lightning somehow birthed a huge purple cube that stood on a hill at the edge of the island’s desert biome. At first, it didn’t do much at all. You could shoot it or swing an ax at it, but the cube couldn’t be damaged. It was just there, motionless, defying explanation and inviting fan theories. Then, it started to move. At specific intervals, which fans seem to have figured out, the cube would roll, just one turn, with an unclear destination in mind.

This isn’t the first huge in-game event in Fortnite. In the lead-up to game’s fifth season, alarms starting sounding off at a mountain base on the island. Eventually, this led to a one-time-only rocket launch, which resulted in a mysterious crack in the sky. Rifts began appearing all over the island, and certain objects — like the sign for a burger joint, for instance — disappeared completely. I found myself checking back in on the game every few days, not so much to enjoy the competitive thrill of battle royale, but to see what had changed. I scanned the sky to see if the crack was shrinking or growing, and I explored towns and forests to check if any notable landmark had gone missing.

But the cube has elicited a different kind of fascination for me. For one thing, the changes are much more obvious. It’s easy to track the steady progress of the cube as it rolls along the map, and you can even see its dark mass as you parachute onto the island. Unlike the rocket launch, which was a blink-and-you’ll-miss-it event, the cube has multiple stages. I didn’t see the first time it moved, but I’ve managed to witness a few rolls since. The recent Android release of Fortnite has helped, as I’m able to check in on the game more regularly to observe the strange object’s movements. I’ve found myself playing multiple times every day, and when I’m not in the game, I’m checking Twitter for new developments.

I have no idea where the cube is going, or what will happen when it gets there. I’m not expecting some life-changing moment when it finally reveals its secret. But the ongoing worldbuilding of Fortnite, as silly as it can be at times, is also incredibly engrossing. It’s a completely different kind of storytelling than we’re used to in video games, especially at the scale of this game, where more than 100 million players are sharing these moments, trading theories and posting videos. Many of those millions are likely ignoring the unfolding narrative, instead focused on getting wins in the competitive arena.

But for the rest of us? We have a cube to follow.