K-pop superstars BTS have gone viral again -- thanks to fan fiction. Not just any fan fiction, though. Outcast is a horror fanfic that doubled as an interactive game on Twitter. The cross-genre phenomenon is the latest peg in the domination of the boy band’s fandom, called ARMY, on Twitter.

Outcast incorporates a fandom-wide choose-your-own-adventure storyline, as fans were encouraged to vote on a poll every night for five nights (along with a bonus storyline on a sixth night), deciding how each cliffhanger resolves itself. It's like Twitch for fan fiction. The game started on Jan. 3, and the bonus night wrapped up on Jan. 8.

Main rapper Suga is the protagonist who, along with best friend J-Hope, downloads a game called Outcast. The two control characters suspiciously named after missing persons (who just happen to be BTS members), and their objective is to get them through each of the five nights alive. The five-night survival structure recalls the wildly popular horror game franchise Five Nights at Freddy’s.

BTS HORROR!AU — in where a game controls lives. hoseok texts yoongi early in the morning about the recent missing case pic.twitter.com/aUZr3pvEs4 — ? (@flirtaus) January 3, 2018

The format is unique for a fanfic as the story is almost exclusively told through dialogue from screenshotted text messages. Writerly flourishes are replaced by the type of deliberate typos that characterize digital speech patterns. Outcast starts with a text exchange between best friends J-Hope and Suga (known in the fandom as Sope) about how someone went missing, which is tied to a video game. The text message format helps the exposition feel more natural and conversational. Occasionally the author, who uses the handle @flirtaus on Twitter, breaks the text message format by incorporating dummy tweets by fictional news outlet National News.

The text message format is also interesting in a fandom where text screenshots are generally used for BTS #imagines where ARMYs can self-insert themselves (as y/n, or “your name”) in romantic scenarios with the members. So, this is a horror story that is seizing on fandom norms and turning them on their head.

The omnipresence of a piece of fan fiction within a fandom is rare, and this is one of those moments it happened among ARMYs. Outcast was so widely circulated that ARMYs made memes about BTS members struggling to get ARMY's attention because they’re too busy reading. This phenomenon also proved that ARMYs have entered the level of fandom where fan theories have become self-sustainable, accelerated by BTS’ short period of promotional downtime.

yoongi logging into the game every night with the same strategy of declining everything #BTSoutcastD4 https://t.co/NeMpc8CNck — joyce (@blooptae) January 7, 2018

The hashtag #BTSOutcast, along with day-specific versions of the hashtag, trended worldwide while the series was live. It’s important to note that BTS and ARMYs are deeply Twitter-centric. After all, BTS was the most tweeted-about artist of 2017. So it feels fitting that a BTS fanfic living on this platform would go viral. The author’s handle was created in early January, and it has already accumulated nearly 421,000 followers.

The story picked up steam between the first and second night -- the first night’s poll was voted on by 14,000 Twitter users, whereas the second night’s poll received 134,000 votes and the third night’s poll got 366,000 votes. The virality of the fanfic spurred countless fan art, reaction videos and even translations.

Fans drew on the mythology surrounding BTS proper to inform their theories of the story’s alternate universe, or AU, even though the author said the story was set in a self-contained universe. There’s an Undertale-esque element of morality to the choose-your-own-adventure storyline as the Outcast game creator taunts readers, “You blame everyone... but yourselves. Aren’t you the ones.. making the choices?” The author later confirmed that ARMY's votes directly impacted the number of survivors when the story ended (SPOILER).

last words until later tonight pic.twitter.com/F6PquZ7y8K — ? (@flirtaus) January 4, 2018

Fan fiction about BTS lives across the web on social media sites like Tumblr and traditional fan fiction hubs like Wattpad, and now Outcast has proven that it can thrive on Twitter. Forbes reported that K-pop’s long history with fan fiction has included idols from groups like SS501 writing their own fan fiction. Recently, Jae from Day6 jokingly added onto a piece of fanfic tweeted at him.