DoomRL, a popular, unofficial take on the classic first-person shooter, is facing a possible shutdown after receiving a cease-and-desist order.

ZeniMax Media, parent company of Bethesda Softworks and owner of the Doom franchise, sent a copyright notice to the DoomRL team ordering it to remove the free download. Co-creator Kornel Kisielewicz tweeted out the letter Thursday night, expressing disappointment.

So... Zenimax have just written to me demanding I take down the DoomRL site... :-/ pic.twitter.com/tXAwdq59Zz — Kornel Kisielewicz (@epyoncf) December 2, 2016

Among the claims is that DoomRL “falsely suggests ZeniMax’s sponsorship or endorsement” due to its use of the series’ intellectual property. If references to Doom are not removed, ZeniMax may take legal action, the letter reads.

That comes as a surprise to Kisielewicz, in light of a recent celebration of Doom fan games. The Game Awards 2016 included Brutal Doom in its round-up of the year’s best fan creations, a nomination which DoomRL players suggested ZeniMax endorsed.

“Zenimax endorses Brutal Doom for [The Game Awards’] Best Fan Project whilst sending a cease and desist to free fan-made DoomRL,” wrote fan Darren Grey on Twitter in a message retweeted by the DoomRL account. “Disgusting.”

A tweet from the official Doom account did encourage voters to throw Brutal Doom their support ahead of the show.

Last day to lend your support to Brutal DOOM 64, nominated for Best Fan Creation at @TheGameAwards https://t.co/4qJaEDSVIG pic.twitter.com/NQKD8nr2bg — DOOM (@DOOM) November 30, 2016

That same category at The Game Awards saw two of its four nominees later removed from the ballot. Pokémon Uranium and Another Metroid 2 Remake, two popular fan games that spent years in development, both shut down after receiving threats from Nintendo earlier this year.

DoomRL remains available to download for now, and there’s even a spiritual successor in the works on Kickstarter. Yet a follow-up tweet from Kisielewicz sounded ominous about the game’s future.

“[Twelve] years modders/fangames kept Doom alive in gamer hearts, kept people waiting for a good Doom to come,” he wrote. “This is the day. You're welcome, Z.”

We’ve reached out to both ZeniMax Media and the pair behind DoomRL for comment, and will update with more when we receive it.

Doom is a popular choice for modders and fans who want to create their own versions of their favorite games. That’s because the game is open-source, its code officially released by developer id Software. The old-school shooter has an active community of level creators, with original designer John Romero himself sharing new free maps to play with the original Windows PC game.