After a series of delays, John Romero's much anticipated Doom Megawad, Sigil, will release on May 31. The free Megawad, which is an "unofficial spiritual successor" the the fourth episode of Ultimate Doom, will be available on the official Sigil website come that date.
The episode is for the original Doom, and is therefore designed to be played with the vanilla version of the game. That means, in the words of Romero, that "no mouselooking or jumping [is] allowed. If you do, you're cheating."
Sigil's delay was caused by unexpected delays in the manufacturing process of the limited edition. That said, Doom community members already have access to the wad, and you can see a speedrun below [spoilers, of course].