Story related and cannot be missed.
This is the final boss of the main game; the infamous Cyberdemon. He is actually fairly easy to beat, even on Nightmare. What you’re going to want to do is to stay as far away from him as possible and keep dodging his missiles, just like in classic Doom. Use smaller enemies to charge the Soul Cube, and then launch it at him a total of FOUR times. Four hits from the Soul Cube will kill the Cyberdemon.
Here is a video of him being defeated on Nightmare by TheGodlikeTube: