SIGIL

SIGIL is the free unofficial fifth episode for The Ultimate DOOM. All the levels were designed by John Romero and the soundtrack was composed by James Paddock. A Deluxe Edition (which costs money) was also released containing a premium soundtrack composed by the band Buckethead. Although it wasn't officially made by id Software, it is supported by them. They even released it as a free add-on for the modern re-releases of the original DOOM.