We had just about lost interest in Fortnite, a mildly-popular PvE building game when it launched, by September of last year. And then it blew up, becoming one of the most popular games in the world in under six months. Even the Toronto Maple Leafs are all about it. And apparently, the catalyst for this dramatic turnaround only took two months to build.

That's according to Epic's Ed Zobrist, who dropped some details on the development of Fortnite’s ultra-popular Battle Royale mode at a GDC panel today. “We started working on this just about the time Save the World was coming out,” he said. “Two months in development, launched in September 26. So let’s do some math: Save the World, the PvE game, launched July 21. [Battle Royale] comes out September 26.”

That’s a total of two months in development, most of which happened between July 25, Save the World’s launch day, and Battle Royale’s September 26 launch day. In order to meet the needs of such a quick development cycle while keeping the PvE mode afloat, some arena shooter experts were brought over to do the bulk of the work.

“And it was the Unreal Tournament team that popped over to pick up the charge for us to basically put originally what we thought would be a PvP version inside our PvE game.”

In a close call, Battle Royale was originally going to be part of the Save the World, an optional PvP mode designed to satiate players looking for some human competition. Because Save the World is still in paid early access, that means Battle Royale could have been stuck behind a $40 wall, which might have slowed its growth significantly. Zobrist said it wasn’t until the final two weeks of Battle Royale’s development that the decision to make it free-to-play and separate from Save the World was made. Nice save.

“I doubt any major publisher could have pulled off this kind of pivot in the time we ended up doing it,” said Zobrist.

Of course, most of the work was already done—Fortnite had been in development since 2011, and as the creators of the Unreal Engine, Epic was one of the best poised studios to pivot like it did. But for a new game mode to be so rapidly developed while differing so much from the game's original vision, and to overshadow the original mode to this degree, is unprecedented as far as I know.