This map, can be played, by selecting the episode "Knee-Deep in hell". I always asked myself "what exacly happened after the portal guarded by the powerful barons in the map E1M8?". The phobos portal anomaly has to be fought from the roots of evil.
The second map, you can play, selecting the episode "inferno city". This new battlefield, is where you will face the final boss.
On Earth, there are still brave Marines fighting, and with their last breath, they will join your slaughter of demons.
Requires Zandronum 3.0 / Gzdoom 1.8.10 / Zdoom-2.8.1