This is one of the most historic events of all time for any of us Doomers: John Romero released a new map for Doom after being absent for 21 years, actually his own replacement of E1M8 for "Knee-Deep in the Dead" (Sandy did the original one that we all know). This is what John says about what to expect:
After exiting the Computer Station you knew the worst was up ahead. You still hadn't reached the place where the demons were coming from. The steel door shuts behind you as you realize you're there; you're at the Phobos Anomaly. Cracks from hell are all over the place as seepage from the portal invades the entire installation. Now it's time to find the portal and stop the demons from coming through. You know UAC had hundreds of scientists working at a high-tech lab somewhere in this area, and the portal must be connected to it somehow. Time to lock and load.
So check the Doomworld thread for a download and enjoy the master's creation, I doubt that this will happen again anytime soon.