Background: Before The Mist

Barovia wasn't always a Demiplane of Dread, of course. What was the valley like before Strahd arrived? The following background is a mix of cannon lore, ideas borrowed from other amazing writers here on reddit, and my own imagination. This is the lore which I use throughout the entire Fleshing Out series.

Druidic Tribes

Long before Strahd's reign and any official settlement, the valley was once home to a collection of nomadic, druidic tribes. These folk lived entirely off the land and were completely unsettled. They are, in fact, the ancient ancestors of the druids and barbarians found in the book. They don't have a collective name for their various tribes, being called either the Forest Folks or the Mountain Folks depending where they live in Barovia. The villagers of Barovia will refer to them as such names or less savory terms.

The Fanes of Barovia

The forest folk of old used to worship a trio of archfey known as the Rozana, informally known as the Ladies Three. The Ladies were extremely powerful nature goddess who watched over the valley. They were:
The Huntress of the Mountain Fane
The Weaver of the Swamp Fane
The Seeker of the Forest Fane
The Ladies, wielded powerful magic, bringing good fortune to those that followed their ways. Only their most devout priests knew the Ladies’ names; to all others, they were known as the Weaver, the Huntress, and the Seeker.
For as long as they ruled the land, known in those days as Cerrunos, the Ladies acted as its sovereigns, making their will known through auguries and omens. Through intermediaries, they wielded powerful magic, but one different from that of mages. They drew power from water and earth, and were bound to the ancient valley in which they lived. It was said that the Ladies could hear everything that happened in their woods, predict the future, twist the threads of human lives and bring blessings as well as curses.

Settlements

As time went by, certain forest folk along with a mix of settlers from other lands established small villages in the valley. These settlements were tiny, each home to no more than a handful of families living in makeshift huts. In time, four major settlements rose in the valley. These were the precursors to the towns in the CoS book. They are: The Village of Barovia, Vallaki, Kresk, and Berez. Although, they were known by other names in the past. Most of these people, settled or not, still actively worshiped the Ladies Three.

The Founding of Delmor

At one point, a little over a thousand years ago, a great circle of mages known as the Delmorians wandered into the valley. These mages had dedicated their lives to studying, understanding, and defeating great evils of a magical nature. In their studies, they discovered the Dark Powers and hoped to somehow subdue the evil gods. The mages were looking for a place to found a base for their research. Not only was the valley of Cerrunos beautiful and rich in magic, it was also remote, flanked by impassable mountains. These mages went on to build the Amber Temple on Mount Ghakis, and subsequently all died at the hands of their own good intentions. The tribes of Cerrunos fought bravely against the mages and their soldiers. Even so, the technology of iron and steel drove the native warriors back to the mountains and wild places, where they hid from the wrath of the Delmoreans. Worship of the Ladies continued, but more weakly than before. Many amongst the tribes believed their failure to be punishment for their failure to serve the Ladies; in their oral histories, this became known as the Second Betrayal.

Invasion of Strahd

Two hundred years later, a haughty warlord named Strahd von Zarovich invaded the fledgling kingdom of Delmor. Weakened by inner conflict and more powerful neighbors, Delmor’s armies fought bravely, but were routed and slaughtered by Zarovich’s superior tactics and arcane magicks.
Fearful, yet optimistic that Zarovich could yet be their savior, the druids of the forest approached him soon after his coronation. They offered him secrets - hidden knowledge that would bind his power to the land, if only he would promise them patronage and safety. Strahd, his interest piqued, accepted.
Thrice did the druids of old Cerunnos guide Strahd von Zarovich, mortal that he was, to the Fanes of the Ladies Three. As he stood in these sanctums and felt their primal, ancient magic, Strahd became determined that these, too, must be conquered. In the Swamp Fane, temple of the reclusive Weaver; in the Forest Fane, home of the wise Huntress; and in the Mountain Fane, home of the mystic Seeker, Strahd re-consecrated each stone altar to his own image, sealing rites of blood and powerful magic that bound his soul to the magic of the Ladies - and so became the Land.
The druids, desperate for a protector, were silent in the face of this profane desecration; the warrior tribes, however, saw it as blasphemy, and separated from the company of the druids in disgust. This became known as the Third Betrayal.
When the Curse of the Dark Powers fell over Strahd’s soul, so too did darkness roll over and corrupt the land of Barovia. The conscious essences of the Ladies were driven to dormancy beneath the shroud of the Shadowfell, and the Land was bound completely to Strahd’s will. Now, the three Fanes lie empty and forgotten, and the druids have given themselves to madness under the influence of Strahd’s dark patronage.
Strahd claimed the valley as his own, renaming it Barovia. He brought another wave of civilization growth to the already established towns and build Castle Ravenloft for his mother. Though Strahd himself was never a terribly religious man, his people and soldiers also brought the religion of the Morning Lord to the valley, which spread rather quickly. Strahd also discovered the Amber Temple in these early days and began conversing with the Dark Power Vampyr.
The Mists Cometh
The rest of the fall of Strahd and Barovia occurs as the book says. The Sergei and Tatyana event pushed Strahd over a mental and emotional edge and he gave into the Dark Power who had become his companion. Strahd became a vampire and the mists cut Barovia off from the rest of the universe.
Remnants of the Ancients
The secrets of the Fanes and the Ladies Three are now lost to time and mist. Even before the citizens of Barovia arrived in the wake of Strahd’s armies, the Delmoreans had already largely forgotten the “savage, primitive” ways of the peoples that preceded them. Only the warrior tribes of the southern mountains, the creatures that guard them, and Strahd himself, who recorded his victories in the Tome of Strahd, still recall the Fanes of Cerunnos.
Only an ancient spirit of wind and sky, a creature named Stribog that once served the Ladies of Cerunnos, still seeks to re-consecrate the Fanes. Stribog is sorrowed by the melancholy that has struck the warrior tribes, the people that he has sworn to watch over; and despises Strahd for the corruption that the curse has wrought upon the druidic people.
Generations past, when a small shrine to the Weaver was threatened by a pack of werewolves, a family of humble farmers defended it. Stribog rewarded these Barovians with the gift of lycanthropy, allowing them and their line to assume the same raven’s form that he took when travelling through Barovia. Under his guidance, the descendants of these farmers - the line of the Martikovs - have come to form the Order of the Feather, a secret society dedicated to keeping the people of Barovia safe from Strahd’s dark hand.
Restoring the Fanes
Dark creatures, tainted by shadow and death, guard these ancient places in Strahd’s name. In the highlands of the Svalich Wood, the hags of Old Bonegrinder desecrate the Forest Fane in the name of the wicked archfey they worship, Ceithlenn of the Crooked Teeth. In the swamps of old Berez, the ancient hag Baba Lysaga watches over the stone circle that once honored the Weaver. Finally, upon the mists of Yester Hill, a of maddened druids nourish and tend to the Gulthias Tree whose roots have infested the forgotten chamber of the Mountain Fane.
So long as each Fane is dedicated to Strahd’s fell personage, the vampire is granted magical benefits in accordance with each location’s patron. From the Forest Fane, Strahd is granted the protection of a nondetection spell. From the Swamp Fane, Strahd gains elemental resistance to fire, cold, and lightning damage. Finally, the Mountain Fane grants Strahd a +4 bonus to his Armor Class. The magic of the three combined, moreover, provides Strahd with a deep bond to the land of Barovia, allowing him to exert his will over its waters, winds, and wildlife.
These Fanes, however, can be re-consecrated, severing Strahd’s connection to their ancient magicks. To do so, each Fane’s guardian must be defeated or driven off, and the Fane itself - a small stone shrine concealed beneath the stone circle that marks it - must be sanctified through a certain ritual. The Huntress’ altar must receive the sacrifice of a predator beast, its organs removed and sealed in stone jars. The Weaver must be provided an offering of fruits and berries, with a mixture of honey and goat’s milk smeared across its altar. Finally, the Seeker also requires blood - but it must be the lifeblood of a mortal, willingly given.
These consecration rites are shown in carvings set upon the stone walls of each Fane’s chamber. Additionally, Stribog bears a fondness for the Martikov line, and may provide this information to the characters if he believes them to be capable warriors with good hearts. Finally, Baba Lysaga, Morgantha’s coven, and the berserker tribes know the secrets of the Fanes.
A creature that re-consecrates these Fanes receives a boon of the Ladies. Not only do the wild beasts of Barovia refrain from harming or hunting them, but the consecrators cannot become lost while ranging in the wilderness, and treat natural difficult terrain as normal terrain for as long as they remain in Barovia.
Repressing the Ladies Three
In the very early years of his vampiric reign, Strahd set about making himself even more powerful, so that no one could challenge his post.
Strahd committed three terrible acts of desecration at the main shrines to the Ladies Three and stole their power over the valley. The Ladies, while still alive at the time of the campaign, are almost completely powerless now.