Toby Fox, the creator of the 2015 indie darling RPG “Undertale,” just released a mysterious new game called “Deltarune.”

It’s playable on PC or Mac and it’s completely free. It features an art style similar to “Undertale,” but right now no one’s sure if this is a sequel, a prequel, a demo, or a completely new and finished project. (It’s probably worth pointing out, though, that “Deltarune” is an anagram of “Undertale.”)

Fox began teasing the game on his Twitter feed on Tuesday. “For those who completed ‘UNDERTALE’ it’s really important that you check @UNDERTALE 24 hours from now,” he said. “I want to make something new, and it all begins with your feedback. A Windows PC / MacOSX PC is needed.”

He followed that up with a series of cryptic tweets, finally ending with the link to “Deltarune’s” website. Fans can download a copy of the game there, which is oddly titled “Survey_Program.exe.” Fox warns some people might run into trouble with Windows Defender when they launch the game, but they can fix the issue by clicking on “More Info” and then “Run Anyway.”

“Undertale” is a charming mashup of role-playing game and bullet hell combat. It’s sold over one million copies to-date and was nominated for numerous awards the year it launched, including best PC game from Destructoid and IGN. Its centers around a child who falls into the Underground, a subterranean world full of monsters, and seeks a way out. To say any more would be to spoil its delightfully metafictional plot.