Enter The Gungeon is a roguelite dungeon crawler that's taking Steam by storm. As players are discovering, it's riddled with secrets, like it's been shot in the head a bunch of times by a gun that fires secrets. Case in point: the mysterious "Case Closed" achievement. (Minor Enter The Gungeon spoilers ahead.)
If you go to Enter The Gungeon's Steam community page, you'll see that Case Closed has no description, and it's only been unlocked by 0.1 per cent of players. That's because it's pretty complicated and, frankly, arbitrary in places. For a while, people didn't even know quite how to trigger it.
However, thanks to the endlessly pulsating MEGABRAIN that is the internet, they have managed to slot together all the pieces. Steam user NeoRizer put together a guide chronicling the assembled findings.
It all begins with one of the game's most basic enemy types: the Bullet Kin. They're walking, sorta-talking bullet creatures with bad aim and dumb little faces. After you've unlocked at least one Enter The Gungeon character's true ending, you have a slight chance of encountering a red caped Bullet Kin on subsequent playthroughs. It can apparently happen pretty much anywhere, but the chances of it occurring at any single moment are very low.
Your first instinct will be to fill the waddling metal stooges with additional lead, but you absolutely should not do that. Instead, you've got to clear out the entire room you're in — every last enemy — without sending the red caped Bullet Kin to the un-fun-geon (read: hell). After ten or so seconds, it will warp out the room. Boom. You did it.
Kidding. You have to do this four more times (five times total). Then and only then will two things happen: 1) you'll unlock the Case Closed achievement, and 2) you'll be able to play as a bullet. Yes, you can become a bullet who shoots bullets at other bullets. The Circle of Video Games is finally complete.
Hilariously, while The Bullet does start with a gun he can use, his default weapon is actually — wait for it — a sword. Specifically, a sword called Blasphemy, because he's a bullet wielding a sword and... yeah, you get it. It functions more or less like Link's sword from old-school Legend of Zelda games. If you've got full health, it fires ranged beam attack and so on. Otherwise, The Bullet's stats aren't anything special, but he's still a pretty great secret.