Your various sources have uncovered the following information on the spirits that Strahd imprisoned in Castle Ravenloft. Once a week, I will allow y'all a check to find out more information on one of the spirits to represent dedicated research by your minions. The group will get to decide which spirits to research.
Amon the Preserver - Information for this dwarf was found in Strahd's own library, written by one of his lieutenants. Not more than a couple of sentences detailing the Ironbreaker clan hidden in the Dreadmount. They were here when Strahd's army invaded to fight off the Beastmen. Amon and his people have a curse upon them, making them near immortal and extremely difficult to fight. When Strahd turned, some of Strahd's own vampire minions even fell to this curse.
Balaur the Soulflayer - Written in Strahd's hand, it details a treaty with a dragon in the far western mountains of Ravenloft. She lives in an area called the Shadowseep, a highly defensible area permeated with negative energy. There is also an accompanying nursery rhyme written in another hand on a torn piece of parchment:
In the depths of the shadows she slumbers
On the land her dark dreams take their toll
When the hour of twilight approaches
She will awaken and swallow your soul
The Slaugh - Provided from Van Richten's archive, encountered both in Ravenloft and in the dread realm of Darkon, the Slaugh are a blight created by Azalin Rex as a weapon against his enemies. They are cursed creatures that reproduce at an alarming rate. They are exceedingly vicious and swarm whole cities, eating any who get in their way.
Malliath Deepking - Written by a long deceased burgermeister of Vallakis, a short written account details a monstrous fish creature that seems to be the lord of the fish monsters in Lake Zarovich. Before he was captured, he would orchestrate raids on the surface for food and slaves.
Alarak the Heir - Information about Alarak comes from two sources, the Vistani and Misha, as he is becoming more accustomed to information through the eyes of Sergei. Alarak was Tatyana's brother. Misha only remembers him as a youth, less than 10 years old. The Vistani tell of Strahd elevating Alarak above even other Vistani and he becoming a terror on his own people.
He Who Walks Behind The Rows - Destroyed when the other creatures were released, He Who Walks Behind The Rows is believed to be a primitive immortal or powerful demon related to agriculture.
Galgann the Doomsayer - You have a detailed record of Galgann, written in a diary in Strahd's library. It is not written in Strahd's hand. It details Galgann, a Nithian Seer who foresaw the destruction of the Nithian Empire at the hands of the Immortals and his quest to save his people. He keeps referring to a soft voice, driving him to the mountains to seek their salvation. There he meets a primitive people from a forgotten Nithian outpost calling themselves the Vistani. They issued him through pervasive mists in the mountains and snuck him into a castle, past it's vampire lord. He met with a being in the shadows that made him an offer. If Galgann would travel to places called dreadrealms and destroy the lords there, this being could save the Nithians.
Contessa Aliana den Solis - Information provided by the Vistani. A Vistani Seer... some say the first Vistani Seer. She foresaw the Devil Strahd and his people come to the castle and the transformation of the realm. Fearing for her people, she took extraordinary measures and sought out deals with dark forces to become a monster capable of holding off Strahd's minions. She sacrificed herself to Strahd and is the reason the Vistani can move in and out of the mists.
Orinonia, the Oblivious Dancer - Information provided in Van Richten's archive. A trapped spirit or prisoner of the Wight Necromancer lord that made Prince Bartok. This spirit was used to torture the necromancer's food before he ate it. She escaped when Van Richten killed the necromancer and scattered it's minions.
Delicae Rayth, the Impaler - Information written in Strahd's own hand. A Heldannic Knight and vampire slayer. Strahd spoke of her highly, more impressed with her skills and dedication. He decided to imprison her instead of slay her, seeing uses for her in the future.
Aurin the Lost Shepherd - Information written in Strahd's own hand. A fairy lord who somehow found his way into the mists. Some say he creates mists of his own, that imprisons people in dreams. Strahd was content to let him live in the forests until an entire village fell asleep and never woke up.
Griat the Skinner - Information provided by Van Richten's archive. A cannibal monster who devoured people and could wear their skins. He originally appeared in Immol, possibly created by foul magics. One of the few times the people of a village risked going to Castle Ravenloft to demand their lords protection.
Torme the Bloodletter - Information provided by Strahd and Misha. Torme was in command of forces who were supposed to reinforce Strahd's army in a battle with the beastmen. Torme had several afflictions that he used leeches to relieve and did not believe he was in good enough humors to lead his forces. Even though Strahd's army won the day, Torme's men did not attack, instead following their lord off the battlefield back to the tower at Krezk. When the dreadrealm was established, Strahd spent an eternity finding ways to torture Torme.