What just happened? John Romero, the legendary game designer and co-founder of id Software, has just released the unofficial spiritual successor to the fourth episode of Doom. Provided you own the original game, Sigil can be played for free.

It was last year when the designer of iconic games such as Doom, Wolfenstein 3D, and Quake revealed "an unofficial spiritual successor to The Ultimate Doom's fourth episode." The announcement marked the original’s 25th anniversary, making those of us who played it the first time around feel even older.

The Sigil add-on pack features nine single-player levels and nine deathmatch levels for fans of the old-school FPS to enjoy. You can also buy a copy bundled with a soundtrack from one-time Guns N' Roses guitarist Buckethead for €6.66 (about $7.46).

There were two physical editions of Sigil, but they sold out a while ago. The Megawad Beast Box and Standard Edition both contain a 16GB USB designed to look like a 3.5-inch floppy disk, much like what Ion Maiden (currently being sued by metal band Iron Maiden) offered in its Founders Edition.

Sigil was expected to launch in February, but production issues with the physical versions saw it delayed by several months. Romero wanted those who ordered them to receive their copies on the same day as the free release.

Sigil can be downloaded for free from Romero’s website. Remember that you need a copy of the original game, which can be grabbed from Steam and the Humble Store for just $5, or GOG for $6.