So a couple of these posts have been rather meta, and not true factions, like the Judge. I had a few options for N, but I kept coming back to something that was not a faction. And since this is my series, I am bending the rules. Again.
For the letter N we have…Noise.
There is a lot you can do with noise. It is one of the best ways to inject atmosphere into the unseen corridors. When you describe a strange sound, and your players ask one another, “What is that?” you are creating the air of mystery that is essential for the old, ruined pile.
Some noises will be easily identifiable, like running water. Others can be identified to a certain point, like footsteps or a scream, though the source will likely not be seen and the players will remain in the dark initially as to what is making the sound.
Whispers are good, too. Soft voices can carry a long way in still halls, and then there is the possibility that the characters are really hearing the whispers in their minds, rather than in the physical realm. Is it the wounded survivor of a dungeon ambush, or the lich guiding you to his tomb? Not just whispers, but voices at normal level can be unsettling in the environment, too. A snatch of mundane conversation, of unseen origin, can give the PCs pause.
A horn blast from faraway can herald a tribe of orcs on the move; are they coming after the party, or another faction? It might be another group of adventurers, trying out an item they discovered. It may be the lone guardian at the door of a sealed tomb, warning the world that a foul being is about to escape. Maybe a ghost is blowing frustration, or attempting to draw the party to it.
That rumbling! It could be that a section of a level has collapsed, or it could herald an earthquake (in which case, the party might have to retreat quickly!). It could be an altar being shoved aside by a massive creature. Or a stone golem dragging its long-ago injured leg across a nearby room.
Music can reach the characters’ ears in the dark. If it is off-key, that can presage either something humorous, like a goblin band trying to get in tune, or something ominous, such as a narcissistic demon lord insisting on a band of demons accompanying him and playing his theme music (OK, that is kinda humorous, as well). It could be a person like Erich Zann from the Lovecraft story.
Or it could be weird and inexplicable, like the wave of a mini-pulsar. This object sits in a chamber deep in the complex, giving off its sound for some reason. Is it trapped here and providing power for a mad wizard’s laboratory? Watching over a sleeping god? The bizarre pulse might get players’ minds jumping to even stranger conclusions.
I remember a lot of the old Judges’ Guild dungeons, like the generic ones in the City-State of the Invincible Overlord, had many sounds written beside the halls and rooms, simple words such as “Groaning,” or “Laughter.” A little bit of this will go a long way in engaging the players and aiding immersion.