What are the dark powers in Ravenloft?
I'm not asking for a list, but rather a description of these entities.
They are some form of superdeities, or perhaps a personification of some multiversal axiom about true evil. They are never described in character/deity terms, and in fact have imprisoned some impressively powerful beings, including a couple deities from other D&D game worlds. (Vecna, specifically, being the most noted, but there was another.)
The dark powers are noted only by what they do, and that is, they reward evil acts with more power, but at a price of less freedom. Sufficiently corrupt individuals find a chunk of the demiplane altered to become their own personal hell... LOADS of power, but trapped forever there, and unable to get whatever it is they truly desired in life.
The Dark Powers also search the multiverse (or at least the Prime Material Plane) for certain "worthy" individuals, and set the mists upon them. If they don't flee the mists, anyone near the victim also is transported. The area is removed from where it was, and attached somewhere in the demiplane.
It is worth noting that they control the demiplane, and can affect all within it. They can not, however, prevent Vecna from granting spells nor communing with his clerics, but they can prevent him from making personal appearances. They have moved regions about, deleted regions, and made and deleted borders on an apparent whim.
The dark powers do not use manifestation forms, do not reveal themselves if they use mortal forms, do not grant spells, are not subject to commune spells. Even calling them entities or an entity is a stretch.
Source Note: It is worth noting that the nature of the mists grabbing parts of the Prime Plane was the focus of one of the TSR Retail Play modules for AD&D 2E; in that same module, PC's who do not flee the mists wind up in Ravenloft as well. The demi-plane does not provide inhabitants; it captures them with the evil being.
In the 3E materials from S&S, it's noted that at least one domain dark-lord was originally from the Demi-plane. Another was in the demi-plane already when the Dark Powers noticed them.
This requires answering a few other, related questions
So I wanted to first spend some time declaring my sources: namely, the 2e books Ravenloft Campaign Setting, Darklords, and Islands of Terror, a selection of Ravenloft novels (including Carnival of Fear, Dance of Death, and others whose titles escape me), the 2e and 3.X Dungeon Master's Guide, the 3.X Manual of the Planes, the transitional module Die Vecna Die!. Whenever possible, I will cite my sources and make every attempt to label personal speculation as such. I will begin with the supplementary questions needed to answer this one:
What is Ravenloft?
The simple answer is that Ravenloft is the Demiplane of Dread (Ravenloft Campaign Setting) but as Pratchett wrote, behind everything simple is a huge trailing tail of complicated. Ravenloft is a 'traditional' demiplane - that is to say, it is part of the Ethereal Plane and has defined borders and boundaries (Manual of the Planes; some demiplanes exist on the Astral plane, but most of these are created as the result of the psionic power genesis) that is ruled over by a mysterious cabal of beings known collectively as the Dark Powers. Ravenloft's boundaries are marked by the Mists, which have the power to reach into any plane connected to the Ethereal Plane that is not otherwise sealed off (Ravenloft Campaign Setting). It is possible to enter Ravenloft by walking through the Mists from the Ethereal Plane, but not generally possible to leave in the same way; in point of fact, egress from Ravenloft is usually somewhat difficult.
How can a character enter or exit Ravenloft?
There are many ways to enter the Demiplane of Dread. The simplest is described above - one can simply walk in from the Ethereal Plane - but others exist. Sometimes, for reasons that are not understood, the Mists will simply reach out and take a being or beings into Ravenloft (Ravenloft Campaign Setting, Darklords, Islands of Terror). This action generally tends to follow one of two observed circumstances; the Mists reach out to ensnare a being of great, passionate, and deliberate evil, or the Mists reach out and snare one or more virtuous beings. In both cases, the victim(s) are usually deposited into a pre-existing Domain, though some exceptions exist in the case of powerfully evil beings (examples include Simon Juste, detailed in Islands of Terror, as well as the domain of Saragoss from the same supplement).
Gates into or out of Ravenloft exist as well, though the only supplement to detail them (Ravenloft Campaign Setting) is sketchy on the details of ingress through a gate. However, it is very explicitly stated that gates leading out of the Demiplane of Terror are rare, tend to only open at specific times or under specific circumstances, and can also be well-hidden and well-guarded. No known cases exist of a Darklord escaping through a gate; in a few cases, a published module ends with the adventurers escaping through a gate. It is, perhaps, reasonable to assume that entering Ravenloft through a gate would be easier than leaving one; however, no canonical evidence exists to back up this statement.
The last, most unusual, and most rare consistent method of leaving Ravenloft involves a conjunction (Ravenloft Campaign Setting); a rare and unusual event in which some part of the Demiplane of Dread simply replaces an equivalent area of the Prime Material Plane, switching places with the two for a brief period. Few, even amongst the most learned on Ravenloft and in the Planes, are even aware that conjunctions occur, and among them no pattern to their happening has been discerned. It is theorized that during a conjunction, even a Darklord might be able to simply walk away and abandon the Demiplane forever, and it is certainly possible (and proven) that other beings can wander out - and in - during that time. When the conjunction is over, any beings on the swapped areas of land travel with that land back to its original home - that is, if part of Barovia overwrites the world of Toril, any beings on the transported part of Barovia go to Ravenloft with it when that piece returns to the whole, and vice versa.
Two unique cases of escaping Ravenloft exist, each of them taken by a Darklord. The Darklord Soth was freed of the Domain of Sithicus when his will was broken by the Dark Powers and he was expelled from the Demiplane (Sithicus still exists and is ruled by a new Darklord) seemingly as a result. Vecna, then a very minor god, was also once the prisoner of Ravenloft, but with the aid of an unidentified outside source was able to trick the god Iuz into entering Ravenloft, rob that god of his divinity and catapult himself into Sigil before achieving true apotheosis. The fallout of the damage wrought upon reality resulted in the switch to 3.0 as the laws of magic and reality were restructured (Die Vecna Die!).
So what are the Dark Powers?
It's unknown, though there are some beings that believe they have the answer. Official supplements make no statements on the true nature of the Dark Powers (though the Ravenloft Campaign Setting contains implications that they may be a group of demigods or minor gods with unknown number, acting in concert), though they have made communications known to some beings either directly (such as to Strahd von Zorovich, the arch-lich Azalin, Lord Soth, and others) or indirectly, through the use of omens. Though one novel revolving around Azalin implies that the Dark Powers are a shadowy cabal of fiends seeking to use Ravenloft to attain godhood, this statement does not match up with their implied and demonstrated power levels and further should be checked against the fact that Azalin was thoroughly insane at the time and thus should not be counted upon as a reliable narrator. In the end, the only reliable statement that can be made about the nature of the Dark Powers is that they are the rulers of the Demiplane of Dread.
However, some patterns can be observed
The Dark Powers treat beings that enter Ravenloft differently depending first on how they entered the Demiplane, and secondly on how they behave once they arrive there. Though no explicit statements are ever made, the actions of the Dark Powers can be observed in the backstories of various Darklords (Ravenloft Campaign Setting, Darklords, Islands of Terror) and through the actions of the Mists, which are under their control. The following can be held as generally true:
The Short Version
Essentially, the Dark Powers are as the Dark Powers do.
The Dark Powers are the entities responsible for making the Demiplane of Dread the miserable, morally grey place that it is. They are the ones that dole out magical rewards to those who seek corruption, and keep the Deathlords trapped in their pocket realms. They're the jailers of Ravenloft, and a stand-in for the DM.