The makers of an online multiplayer game dubbed “Pokémon with guns” say it’s sold five million copies in three days.
Palworld, from Japanese company Pocketpair, has been an overnight success since it launched on Friday.
But its surprising popularity meant players struggled to log on at times due to server crashes.
The game’s also been criticised as a Pokémon rip-off due to the similarity of its creatures.
Palworld is described as a “multiplayer monster-collecting game with open-world survival elements”.
Players, known as “pal-tamers” travel around a large map battling human foes and creatures known as “pals” which can be captured and recruited.
The monsters can either fight alongside the player in battles, or be put to work at a base, crafting supplies and items for use in the field.
Wesley Yin-Poole, UK News Editor for gaming website IGN, said early interest in the game “gave us an indication that it was going to be very popular, but no one expected it to be as popular as it has proven over the weekend”.
He said Pocketpair is a small developer “with a few games under its belt, but nothing explosively popular like this”.
“The Last of Us Part Two sold four million copies in three days. Palworld has sold faster than the fastest-selling PlayStation exclusive of all time, the success is astronomical.
“It just goes to show that this sort of genre is immensely popular and, personally, I’m just surprised it’s taken this long for someone to do something like this.”
Despite its huge sales, Palworld has not been a hit with everyone.
Its character design in particular has come in for criticism, with people accusing its developer of copying designs from Pokémon.
Reviewers and fans have also pointed out that Palworld’s gameplay borrows heavily from survival games like Rust and Ark: Survival.
However, it’s been praised for its graphics and has a “very positive” user rating on digital game store Steam, based on 40,000 reviews.
Wesley points out that it’s “impossible to get away” from the similarities with Pokémon but he doesn’t think “it’s necessarily problematic from a legal standpoint”.
“While it has got monsters that are very similar to Pokémon, the way the game plays is very different,” he tells BBC Newsbeat.
“What’s really impressive about it, is just how well it’s taken ideas from other games and blended them together to create this really compelling mishmash of concepts.
“So I think that’s more about what’s happening here than simply just ripping off Pokémon.”
In a message posted on Monday morning, Palworld’s developers said their support team had received 50,000 inquiries and was “aware of serious bugs” in the game.
The game has been launched in “early access” – where the version available to players is not the final product.
Fans get to play the game sooner than they might have, and companies are able to raise money towards polishing the game.
But an unfinished version of the game can mean players experience bugs, crashes and other problems.
Pocketpair said it would “share information about the fixes for these issues as soon as possible”.
Charlotte, who streams on Twitch as CharlotteMarieUK, says she’s been playing with friends and noticed issues with lags and the game freezing.
“I think it’s probably a bit overwhelming for the service at the moment. But I definitely think that’s going to improve,” she says.
And it hasn’t stopped her getting hooked.
“Really, everyone’s wanting to play it right now,” she says.
“When I get a game like this, it just makes me want to play all the time. I go home, jump on with friends. I’m really looking forward to seeing what they do with it.”