Flesh and blood can do nothing, sir, against devils; and that's what's in the Grey Room after dark.

-William Hope Hodgson, The Gateway of the Monster

Few hunters, no matter how successful and persistent in their careers, ever have the misfortune of encountering one of the fell creatures known as fiends. These infernal monstrosities are evil to an extent that most can't imagine, so much so that they warp the fabric of reality around themselves.

They are the embodiment of everything Van Richten spent his life trying to combat, yet with all his knowledge, skill, and preparation, even he was overmatched in his lone confrontation with one of these entities. Do not lightly seek them out, for to fall into their clutches is to risk not only one's life, but one's very soul.

Transposition

Van Richten uncovered two ways in which a fiend might find itself in the Land of Mists. The first of these is the process known as transposition, in which a fiend slowly trades places with a mortal already within Ravenloft.

Fiends appear capable of sensing the thoughts and feelings of mortals, even from another plane. They are particularly able to detect thoughts of evil and wickedness, and they are drawn to these as a moth is to a candle. Fiends appear to feed in some way from the wickedness of mortals and a fiend who manages to find a budding source of evil seeks to bring it to fuller bloom. A psychic link is formed between the fiend and its unwitting vessel, and the fiend uses this link to encourage the mortal to greater and greater acts of infamy. As the mortal succumbs, the bond between the two grows ever stronger. The mortal begins to physically change, increasingly resembling the fiend. This is an extremely painful process. The fiend's lust for continued malevolence eventually proves it undoing. If it continues to spur the mortal on, it eventually takes the mortal's place in Ravenloft. The mortal is trapped in whatever infernal realm the fiend came from, while the fiend is trapped in Ravenloft in turn.

An individual who fails a Powers Checks might find themself transposing with a fiend in lieu of the usual consequences. Transposition occurs in five stages, with each failed powers check moving the victim to the next stage. The exact effects of each stage can vary depending on the type of fiend involved, but all transpositions have the effect of granting fiendish characteristics to the afflicted.

At stage one, the psychic bond between the fiend and victim is formed. This stage is accompanied by minor physical changes in the victim, which can usually be disguised with some difficulty. A victim transposing with a nabassu might find that his eyes have turned entirely gray, for instance, while one transposing with a succubus might actually become more physically attractive, with wrinkles smoothing and blemishes vanishing. Some minor aches accompany this stage, but they're not debilitating.

In stage two, the victim suffers a glaringly obvious physical change, such as gaining scales or horns, and the pain increases dramatically. So great is the pain that the victim suffers a -1 penalty to all rolls.

In stage three. the transposition begins in earnest. The victim is considered a tiefling, for all intents and purposes, and their fiendish characteristics are unmistakable. They might have scales from head to toe, or small, nonfunctional wings sprouting from their back. Change the victim's race to 'tiefling' and apply changes as normal.

In stage four, the fiend has nearly taken the victim's place. The victim is now considered a cambion. The pain is so severe that the victim now has disadvantage on all rolls.

In stage five, the mortal is doomed, and the fiend entrapped. The mortal vanishes to the infernal realms, and the fiend takes their place in Ravenloft.

A victim can be saved from transposition up until the fourth stage, though this is a difficult prospect. Some spells can aid the victim. If a cleric native to Ravenloft casts Dispel Evil and Good on the victim, the fiend is permanently forced from the victim's body. The victim may not survive, however. They must make a Constitution saving throw against your spell save DC or die, though such a death is certainly preferable to the fate that otherwise awaited them. The spell banishment can also drive the fiend away, but it gets a save as normal. Unfortunately, even if successful, the victim retains all physical changes. A stage three victim will have their race switch back to its original, but their physical appearance will remain the same. Neither of the above spells can save a victim from the fourth stage of transposition.

Conjuring

A more common way for a fiend to enter Ravenloft is through magical conjuration.

The gate spell is an example of how one might be magically called to Ravenloft. When the conjured fiend finds that it can't return to its plane of origin, it's likely to be incenses. The summoner had best have magical means of compulsion ready, such as a planar binding spell, though it would be far wiser not to conjure a fiend at all.

Reality Wrinkles

The very presence of fiends has a warping effect on their surroundings. This warping is called a reality wrinkle. The sheer, undiluted evil of the fiend reacts with the Mists and creates what is essentially a mobile pocket domain centered on the fiend.

The size of the reality wrinkle is directly related to the power of the fiend. Powerful fiends, such as pit fiends and balors, have incredibly large reality wrinkles, while weak fiends such as imps might have reality wrinkles that barely extend beyond themselves.

As mentioned above, the reality wrinkle serves as a pocket domain, with the fiend itself as lord. Unlike a true darklord, however, the fiend is not cursed, and is free to wander within Ravenloft as it wishes. The reality wrinkle moves with it, and wherever the fiend travels, it temporarily usurps control of the land around it. This means that the fiend can't be hindered by the closed borders of a domain, for its reality wrinkle supersedes those borders. Nor can a darklord exert any of its granted powers within a fiend's reality wrinkle. Darklords are immediately aware of any fiends within their domains, as they can feel the sudden severance with a portion of their land.

Power Rituals

Under normal circumstances, fiends exert no special powers within their reality wrinkles other than the above. There is a method, however, known as a power ritual, for the fiend to gain such powers.

A power ritual is an arcane rite, known only to fiends, that enables a creature to draw great power directly from the land itself. The exact nature of these rituals is unknown, and each one seems to be unique. A fiend can only ever attempt one power ritual in each domain. After that, whether success or failure, the fiend is unable to draw further power from that land and must try another domain in its place.

If successful, the fiend's reality wrinkle immediately halves, and its bond to the Land of Mists grows stronger. The more power rituals the fiend succeeds at, the more powers it possesses, but the harder it becomes for the fiend to ever escape Ravenloft.