The new Doom does a pretty decent job of bringing back some of the difficulty of 90's era games. Of course, they offer up their own increased difficulties should you want a harder challenge. Nightmare is a pretty tough mode but manageable with some practice and skill.
However, it's the Ultra Nightmare mode that id Software kept flaunting as their end all be all difficulty. A game mode so hard that none of the actual developers were able to finish the game while playing it. What made Ultra Nightmare truly difficult was the fact that it was a single life mode. Permadeath in all its glory. You could be hours into a run when one slip up happens and your day was just wasted. Start back over again. Break a controller. Crack a keyboard over your knee.
Proving that players will always break, beat or crack open parts of a game devs spent YEARS working on as soon as you release it, Youtuber Zero Master managed the complete Ultra Nightmare mode just one day after the game was released!
Check out his whole speed run up top and take notes.