Multiplayer roguelike Risk of Rain 2 has attracted over 650,000 players during its first week, thanks in part to developer Hopoo's buy-one-get-one-free sale. 

The offer gifted players who purchased the $19.99 Early Access title within 48 hours a second copy of the game that could be shared with a friend. 

It was a smart way for Hopoo to generate interest in the game and ensure those early players would have a pal to adventure with, and it evidently paid off. 

As part of the sale, around 150,000 players had the opportunity to bring another friend into the fold, and the sequel is now on track to cross 500,000 sales by the end of the week -- though it's unclear exactly how many of those came through the promotion.

"I just wanted to say from the whole team how absolutely floored we are with the reception so far," said Hopoo Games co-founder, Duncan Drummond.

"I was personally hoping for something to match at least half of the original game: apparently we’re doing a lot better. It’s absolutely surreal -- thank you all again."