Update: Our guesses yesterday were more or less spot-on. Sigil is a Doom megawad, due in February 2019, and it's been announced on today's 25th anniversary of Doom's original release. It's an 'unofficial spiritual successor' to Ultimate Doom's fourth episode, and features nine singleplayer levels and nine deathmatch stages.

I'm excited to announce the project that I've been working on: SIGIL, an unofficial spiritual successor to The Ultimate DOOM's 4th episode! Read about it here: https://t.co/bVDrRntSqG #johnromero #romerogames #sigil #doom#pcgaming #pcgames #buckethead #retrocollectionDecember 10, 2018

You'll need the 1993 original to play it. Meanwhile, there's a ludicrous collector's edition from Limited Run Games to go alongside:

Here are some more screens:

Original story: Doom designer John Romero has finished work on his next game, and will fully reveal it tomorrow. He's teasing it with the slogan "Prepare for 5"—that could be a reference to the fact that Doom, combined with its updated version Ultimate Doom, was made up of four episodes.

We also know that Romero, one of the co-founders of id Software, is working on a new shooter and has been since 2014. He left id Software in the '90s, and the new shooter is being put together by his own company, Romero Games.

In an email sent to fans (view a screenshot of it here), Romero said that pre-orders would start tomorrow, and that the reveal would be related to recent teases from his Instagram page, which is full of images of the number five, like the one at the top of this post.

Whatever it is, we don't have long to wait—check back in tomorrow for more details. The reveal will mark the 25th anniversary of the iconic FPS's release.