Quake: How to Activate Nightmare Mode

In Quake, Nightmare Mode is a special, more challenging difficulty that reduces your maximum health to 50 (meaning that health pickups will only heal you up to that amount) and modifies enemy behavior to be more difficult. Beating the game in this mode used to only give you bragging rights, but now the 2021 remaster features achievements that can only be unlocked by playing the game in nightmare difficulty.

First, choose any difficulty from the starting room. It doesn’t matter if you enter the Easy, Normal, or Hard portals, as they all lead to the same map. Next, you’ll want to enter the room for Episode 4: The Elder World, which should be to your right.
Jump into the pool of water, but don’t fall all the way down as if you were starting the episode normally. Instead, swim under the overhang closest to the entrance of the room, look for something that resembles a raised platform below, and align yourself with it. The ledge can be difficult to see from the water, so make sure to practice this a few times to get a feel for it.
Drop yourself onto the ledge. If done correctly, you should be here:
Carefully make your way to the passageway to your left, and enter the red portal to activate Nightmare Mode.
If you didn’t succeed at falling onto the wooden ledge, you can always try again by entering the portal at the back of Episode 4’s room, which brings you back to the level selection area.