DOOM Zero

DOOM Zero is a fan-made map-pack/mod by Iruka and Red Pass Games. It is a prequel to DOOM II and is designed to show the limits of what you can do with vanilla DOOM II. It was released to celebrate the 25th anniversary of DOOM II's release. Although it was not devolved by Id Software, it is officially supported by them. They released it as a free add-on on December 15th, 2020 for modern re-releases of DOOM and DOOM II.