The Anomalous Subsurface Environment has no villainous mastermind running the dungeon. It's got villains, some of which are undoubtedly masterminds, or at the very least Hive Minds, but they didn't make the dungeon, they don't run it, they just live in it. None of this is particularly safe to bring up in conversation with aforementioned villains, but that's the way it is. The ASE is a metaphysical space realized in stone, trap, and monster.
As I mentioned in last night's post, the dungeon, by necessity, has secret levels of clockworks operating its traps. These need to be maintained by someone. The simplest solution would be for the dungeon would maintain itself, simply refusing to obey the law of conservation of mass and energy. While that is true, a megadungeon has a personality, and expresses that in how it goes about maintaining itself. This personality is expressed as the dungeon elemental.
I'm not a fan of the elemental planes, or any of the traditional D&D cosmology. I like the notion of elementals as material spirits, drawn from the surrounding environment. In that tradition, the dungeon elemental would be a spiritual expression of the physical dungeon, representing its metaphysical will. On the other hand, an Elemental Plane of Dungeons would be pretty cool.
Do note, that the dungeon as semi-sentient force is, of course, a metaphor for the Dungeon Master himself. So another way to look at the Dungeon Elemental is as a DM Elemental.
Dungeon Elemental, Lesser
No. Enc: 1 (3d6)
Movement: 120' (40')
Gaseous: 90' (30')
Armor Class: -1
Hit Dice: 6
Hoard Class: XVII
The dungeon elemental, or "Door Closer" as it is referred to by the few adventurers who have encountered them, is a physical expression of a dungeon's will. It performs basic upkeep on the dungeon when there are no intelligent entities to do so, ensuring that traps are in good working order, doors continue to function relatively smoothly, the air is ventilated enough to support the dungeon ecology, and that vital support columns and buttresses are in no risk of collapse.
The elemental appears as a red humanoid, extremely thin, wiry, and flexible, with sharp black claws on its hands and feet. It has no ears or nostrils, just a pair of empty eye sockets and a lamprey-like mouth, with nested circles of sharp teeth. It has no bones, and is able to compress itself and squeeze through incredibly tight spaces. The elemental may become invisible three times per day, and may assume gaseous form twice per day (for 6 turns duration).
Dungeon elementals can only be damaged by magic weapons.
Encounters in a normal dungeon level with a dungeon elemental are brief and from a distance. The elemental uses its powers to remain unobserved, and it will do its best to avoid combat, or interaction of any kind, with any dungeon inhabitants and/or intruding adventurers. A sighting of these creatures is rare indeed, and few are the experienced adventurers who speak of seeing the strange red ghost moving swiftly through the dungeon, closing doors and repairing broken traps.
The dungeon elementals spend most of their time in secret levels between the "normal" dungeon levels. These secret levels contain the massive flywheels, gears, and chains used to operate the dungeon's traps, systems for air purification, pumps to move water through the dungeon, and other equipment required to keep the dungeon a suitable environment for the bizarre menagerie of monsters that make it home. The secret levels have powerful dweomers preventing detection of these hidden spaces by normal or magical means. Should an unlucky adventuring party accidentally find their way to one of these secret levels, they will be mercilessly attacked until dead. The dungeon wants to keep its secrets, and dead men tell no tales.
The elementals are intelligent, but have no will or desire of their own. They exist only to serve the dungeon.