Spawn is a five-level minisode (four maps and a boss closer) for Doom II to be ran in GZDoom, released at the tail end of 2011 by Dave Billing. Unfortunately, it received very little hype on its debut. I say that because it's a fantastic little mapset. SPAWN takes place at a UAC xenobiological research facility on the planet Epsilon Eridani III (more than ten light-years away from earth). As is usually the case, something has gone horribly wrong at the base and it's up to you to sort things out so that it never so much as touches home. You have to penetrate deep into the heart of the base and eliminate the power core in order to halt this particular invasion.
