The Sixth House, also known as House Dagoth, is not a Great House in the modern sense but a long-buried remnant of the ancient Dunmer past. It traces its origins to the time before the Tribunal, when House Dagoth fought alongside the other Houses at the Battle of Red Mountain. Thought destroyed in that war, the House instead went underground and has reappeared in the modern era twisted and reborn under its immortal leader, Dagoth Ur.
Unlike other dungeon types, the Sixth House squats in other dungeons to make them their home. Most often caves, but Dunmer Strongholds and Dwemer Ruins are also frequent hosts for Sixth House bases. These bases should be perversions of the base dungeon type.
Sixth House bases serve as holy sites, bases to train cultists, and generally places for cultists to become deeper enmeshed in Dagoth Ur's dreams. There should not be any inhabitants of any Sixth House bases unaffiliated with the House, except for prisoners, of course.
Corprus Creatures - The Sixth House is responsible for Corprus disease, and uses it to create monsters to help further spread the disease as well as control victims that Dagoth Ur does not see as being fit for elevation into an ash creature. Corprus Creatures are mindless and can be found wandering aimlessly around the bases.
Ash Creatures - the Sixth House has a multitude of stages a cultist must go through after their initiation; Ash Zombies, Ash Slaves, Ash Ghoul, culminating in the transformation into an Ascended Sleeper. These are actual members of the cult and, except the zombies and slaves, can even be named, as a result they can be placed in more prominant locations in the dungeons and implied to have specific duties.
Dreamers - Ordinary dunmer who have fallen victim to Dagoth Ur's dream sendings. They still appear normal, having not yet developed Corprus, or at least having not yet let it mutate them noticeably.
Daedra - In rare cases, primarily in the Sixth House citadels on red mountain, the Sixth house has been known to summon daedra to help guard their facilities.
Since every Sixth House base serves as a recruitment center, and a holy site we have a handful of unique Sixth House clutter pieces, banners, etc.
Sixth House religious practices include ringing lots of bells, the exact purpose of which is unknown, but we have a wide variety of bells that can be sued to decorate their hallways, chapels, etc.
Cultists also seem to take a sort of dark communion by ingesting the flesh of their Corprus monsters living in their bases. Virtually every base in the original game had one or more troughs of corprus meat hunks that they implied were eaten by the cultists.
Lighting for the Sixth House is more stylized than virtually anywhere else in the game, being a sinister, bloody red. We have numerous red candles specifically for the Sixth house. We also have special braziers made just for our Sixth House bases, normal fire light is also acceptable for lighting interiors.
Ash statues should feature prominently in the decor as well, these sinister little artworks broadcast and extend Dagoth Ur's dream sendings, and are a primary method of recruitment and keeping cultists under control until Corprus is able to take hold.
The Sixth House has a range of peculiar and unique magic to use when trapping their bases
Blight clouds - these should be used sparingly, appearing near entrances. Keep in mind that the player becomes immune to the blight about a third of the way through the main quest, so these clouds are for flavor more than anything, but can be used on certain major locations to discourage (but not prevent) players from venturing into late-game Sixth house bases early (such as the citadels, or Kogoruhn)
Barriers - come in two forms, a dome or a wall. These are simple obstacles to prevent players from progressing until some combat challenge is beaten, or a trigger is set.
The previous two traps can usually be resolved by finding and removing a special ash statue
Rune Traps - Given that House Dagoth is fond of magical solutions to problems, using rune traps is perfectly acceptable.
Bell Strings - These serve as a warning system for the cultists and will alert any cultists in the area if the player collides with these bells.