This article was originally published on April 17, 2026 - read the full issue

By Alex Spencer

Edmund McMillen has a few million-sellers under his belt. 2010's Super Meat Boy and 2011's The Binding Of Isaac both hit 1m sales around the one-year mark. But his latest game, conceived back in 2012 and developed over the past six years with longtime collaborator Tyler Glaiel, has outstripped all of those previous successes. Released on February 10, Mewgenics hit the 1m milestone within just seven days.

"I've been asked in the past: what do these numbers feel like? And I'm always like, 'I don't know!'" McMillen tells Knowledge, a few weeks on from launch. "It's so hard to put in words."

But here's his current best attempt: "Your happiness only goes up to ten, right?" McMillen lives a pretty nice life, he says, so he spends a lot of time at the upper end of that scale. "And through the two weeks of launch, I was feeling at a ten, but I can't go higher than that – even though you're getting all this stuff that you'd assume would force you to break the cap and launch into space."

Coming back down to earth

Having gone through this process a few times now, though, McMillen reckons that "behind the scenes" there is some emotional recalibration going on, which can make it difficult to return to normal. "Once the dust settles – I've seen people beat the game, hit all the beats and whatever else – I'm alone with my thoughts," he says. "And the drop, for me, was considerable. I felt like how people describe going to raves for 48 hours, where they don't sleep and just do molly, you know what I mean? Food didn't taste good. Videogames weren't fun. I didn't want to hang out with my friends. I didn't want to do anything. And it felt like that for a week and a half. But it could have been worse. I've felt worse in the past with this sort of stuff."

Throughout all of this, there was at least one positive thing that McMillen could cling onto: what this success meant for his development partner. Glaiel himself acknowledges, "I've not made anything remotely close to as big as this." (McMillen and Glaiel's previous collaboration, 2017's The End Is Nigh, sold 100,000 copies in its first year.)

"I've actually gotten more joy from seeing Tyler's life change because of this than I have from anything else – the reviews or the numbers or the money," McMillen says. "Because that's this thing I can see. It makes sense to me." As we speak, Glaiel is in the process of buying his first home, something only possible because of Mewgenics' success.

Holding up to stress-testing

But when a game's core dev team consists of just two people, who are also self-publishing, hitting these numbers does come with its challenges. Relative to the team size, Mewgenics went through a lot of testing – "a year and a half of private beta with 30 or 40 people playing regularly, plus a bunch of automatic testing that would play the game rapidly overnight," Glaiel explains – but all that time paled in comparison to the reality. "It was something like 30 million hours of people playing the game in that launch week. And so everything that could possibly happen happens."

On top of that, Glaiel was nervous to make changes while the game had so many active players: "If I push a patch that crashes during the tutorial or whatever, it collapses. Everything falls apart." But, looking back, he's proud of how well the game has held up to millions of hours' worth of stress-testing: "I've had to fix maybe ten large bugs – like, crash bugs – out of that many hours of people playing. There's a bunch of other little bugs that show up. But I feel like we did a pretty good job, considering how complex and large the game is."

Of course, for a game like this, bugs aren't the only concern – there's also the matter of players who want to unpick every design decision the team has made over the past six years.

"This is the first game I've made where people have opinions on it," Glaiel laughs. "I mean, like, The End Is Nigh, people have opinions on that, [but] there's no expectation that we would update the game with rebalanced levels or enemies or something. People might hate the birds, but they just learn to deal with them. With Mewgenics, people have really strong opinions on what we should or shouldn't change."

Processing a deluge of player feedback

Glaiel acknowledges that the sheer amount of feedback they've received from players "can get overwhelming at times" – especially once you start to factor in the game's most ardent fans. "It's so bizarre," McMillen says, "when people reach out and say, 'Listen, I love this game. It's my game of the year, best game ever… except, it really falls off after the 500-hour mark.'"

These players are "a tiny fraction" of the game's audience, Glaiel says, but also the most invested. "Whenever you read feedback from people on Discord and Reddit, it's those people who've got super into the game." The developers don't want to ignore that part of their audience, then, but it's hard to design for players who've min-maxed the game's breeding system to produce an army of perfect clones. "There's not a lot we can do to support that, because at that point you can basically do infinite damage," Glaiel says. "And we can't just make a boss that has infinite health… Or can we?"

McMillen and Glaiel say they try to examine any issues that have been flagged, but without simply taking the offered diagnosis of what's wrong – or how to fix it. "This is game design 101 stuff," Glaiel says. "This is what they teach people in school." But there's another reason to do things this way, McMillen adds: "Publicly, it is very bad to appear to bend to the will of someone online. Which sucks in a lot of ways, because sometimes they might be right!"

What happens next?

If The Binding Of Isaac is any indication, Mewgenics has a long life ahead of it, and we'll have to wait and see whether the developers ever put a paw wrong. One thing McMillen says he's learned from supporting the former game for so long: "For every nerf, have two small buffs. As long as there's some counterbalance there to appease the fans, I think that's a pretty good solution."

For now, McMillen is confident. "I'm going to go on record saying that I think we did a fantastic job balancing this game before release," he says. "I don't think the changes that we're going to make for balance are going to be Slay The Spire-style changes that will polarise..." Glaiel cuts in: "It's still gonna piss off the whole Internet!" The pair argue the point back and forth in a way that speaks to all their years as friends and collaborators.

There's one thing they can agree on, however: the best part of game development is the actual development part. "I think both of us enjoy making games," McMillen says. "Releasing games kind of sucks." The money is handy, of course, but McMillen's already had enough success to live a comfortable life, and he's clear about his priorities: "Let's make another game!"

This article was originally published on April 17, 2026 - read the full issue