Piratecat, you might recall that I have a recurring theme in my game that involves the Mother of Dreams, a psionic dragon who was infused with demonic and celestial energy, which caused her dreams to manifest as semi-corporeal creatures. This young dragon girl, named Trilla, fled deep into the tunnels beneath the earth, where now her nightmares roam unbound.
I named these nightmares the Trillith.
Most of the dream incarnations the various parties of my setting have encountered have been based on specific concepts, like Deception, Vengeance, Agony, and Helplessness, each with trappings specific to the nightmare that spawned them. They have mastered the methods of shaping the dreams of their mother, so that each night they can create new allies of their choosing, though Trilla resists with what little will she has left. Sometimes she dreams of hope, freedom, or justice, and some of these dreams manage to survive the rigors of their more cruel kin.
Trillith dislike the physical. Normally they exist as intangible psychic presences, able to affect the world through telepathy and telekinesis. If there is appropriate material nearby, they can animate it into a form that matches their nightmare aspect. These are often a reflection of Trilla herself, so they tend to forms that are vaguely draconic, or that are mockeries of how she perceives herself as a young Elvish girl, even though now she is thousands of years old, kept alive by her dreams.
Weak Trillith are usually bound to a single physical form, and they lack strong psychic abilities, though their appearances are often the most distrubing. They focus on flesh-shaping just as their stronger kin perfect dreamshaping. Greater Trillith seldom assume physical form, and are treated almost as gods by lesser Trillith.
In general, the Trillith are a mix of the powers of elementals and outsiders, and occasionally undead. Whenever you defeat a Trillith (whatever 'defeating' means), you might gain a bit of freedom from whatever nightmare it represents, or if it is a positive dream you might gain some of its strength. When you slay a Trillith, the psychic energy of its death floods into your mind, and if the weak-willed will be tainted by the Trillith's aspect.
One example of the Trillith show up in my Mother of Dreams storyhour (others will show up later, of course). And in my last campaign the party dealt with several Trillith, primarily stronger ones with a specific aspect.
Deception was the most refined of the Trillith, playing something like a classical Satan from Milton. He could project illusions of appearing as a personable old man, and it was almost impossible to not believe his lies. When forced into a physical form, he was nothing but shadows and writhing tendrils.
If you've seen The Grudge, you know what Agony sounds like when she moves.
The strangest one ever was The World Falling Upon Itself, a Trillith that evoked complete claustrophobia in those around it. It could grab people and drag them xorn-like into the earth.
There were others, tied heavily into the specific events of the games. One element that encouraged the Trillith as intriguing villains was the Song, created by the Worldshaper Worm that carved the tunnels beneath the earth. One of the great elemental spirits, it possessed sway on all things physical, and it created a song that could drive the psychic Trillith into a physcial body. It taught the song to Trilla, and to fey native to the land below, and it was the first weapon against their tyranny of dreams.