In DOOM 2016, players could dial up the difficulty to Nightmare in order to give themselves a real challenge. That option will still be in DOOM Eternal, but it's no longer the highest option players have to choose from.
The dev team has confirmed that there will be a level higher than Nightmare made available, and it even includes some unique twists to gameplay. This mode starts off with the difficulty of Nightmare, but adds in some roguelike features to the experience. For example, you'll only have one life to make it through the entire campaign with. You'll have to do that without any checkpoints as well.
There's no hiding your performance in this new difficulty either. Other players will be able to see how far into the experience you made it before you kicked the bucket. That's an extra bit of incentive to keep revisiting the mode and one-upping your previous performance.