I have decided to collect the various books of the Plague Tale franchise and to gather all interestings informations I could find. Please, note that English isn't my native language so forgive me for the various mistakes that you will probably have to read. Now, let's start :

Origins
- Prima Macula" means "First Stain", referring to the stains that appear on the wearer. The first was born at an uncertain time. Long ago, in ancient times, Greek alchemists managed to convince local lords of the seriousness of the Macula and that it could destroy humanity. They obtained the necessary funds to create the Order and will subsequently gather the evidence of previous carriers and be ready to act when the Macula awakens. They settled on La Cuna around 400 A.D. This allowed them to isolate themselves to receive and study the carriers. The location of the island meant that it was often disputed, which explains the Byzantine and Moorish architecture.
- A young boy, Basileus (or Basil), born in 534. He is not related with Hugo. His seizures and marks lead his parents to abandon him in front of a church. Adopted by monks, he is victim of mistreatment. At one point, during one of his seizures, the floor of the church where he lives begins to tremble. Rumours of a demonic child begin to circulate and reach the alchemists of the Order. They adopt him and bring him to La Cuna. Well treated, he is given a close guard including a mercenary of modest origin named Aelia Dragos. A strong bond develops between them. A carrier and a protector do not have to be of the same blood. With the well-being of one guaranteed by the other, Basil's life improves and he manages to cross the various thresholds.
- The evolution of the bearer makes the alchemists think that he is about to cross the last one. Under pressure from some members of the Order and their patrons, Basil is isolated and locked away deep in the mountain. The protector will write on a stone that the Order cannot heal the carriers and eventually kills them. She will try to reach Basil and will fight a furious battle but will die on the way. The alchemists have thus triggered the final threshold by separating the two, which will trigger a rat invasion, earthquakes and the Nebula. The Nebula takes the form of a thick fog that hides the sun and allows the rats, no longer limited by its light, to thrive. This will trigger Justinian's plague.

The cult of the Child
- Over time, the Order's presence on the island gradually fades. The link between its activities and the epidemic has never been revealed, and the experiments that were conducted there in secret will give rise to a legend: the cult of the Child. A myth about a young boy who was born during a dark time and was able to emit a glow that enraged the creatures of the night, who locked him up deep in their lair. A young female warrior tries to save him but fails. However, this causes the child to emit his glow one last time and restores the fertility of the land.
- On the island, many inhabitants are sterile, probably due to the influence of the rats and the Macula. Among them are the count and the countess. The count proclaims himself a descendant of the warrior. He will create the cult 20 years before Innocence.

Innocence
- Beatrice seems to be of modest origin, the Requiem artbook describes her as a noblewoman "by marriage". She was a highly regarded alchemist who lived a courtly life before devoting herself entirely to alchemy for healing Hugo.
- Amicia probably did not spend much time with her mother, who was busy with Hugo's illness. This affected her childhood, which led her to seek out the company of those closest to her. Beatrice will work on an antidote for 5 years, after the birth of her son.
- After the early loss of his parents, Lucas was trained in alchemy at an early age. He was then hired as an apprentice by the master alchemist Laurentius.


Tenebris
- In this book we learn that Hugo's Macula and nightmares can alter the reality around him and cause hallucinations or contagious emotions. The story begins a few weeks after the fight against Vitalis. Amicia, Hugo, Beatrice and Lucas travel through the countryside. The group finds itself pursued by a band of flagellants. Hugo is captured while trying to protect Brume, a mouse he has adopted. It turns out that their attackers are convinced that Hugo is the child of suffering, a future martyr destined to suffer for the sins of humanity. The latter has a seizure which triggers an invasion of rats. Amicia manages to shoot a stone that tears off the nose of the leader of the gang and then knocks Hugo out.
- The latter falls ill. He suffers from Contra Imperum, a kind of counter-attack of the Macula which results in fever, fatigue and seizures. Mention is made of the Sanguinis Itinera (which translates as 'Journey through the Blood'), a grimoire containing information about the Macula. Hugo seems to be able to take control of the rats in his dreams.
- It turns out that a monastic order led by a certain Olivio has a large library where the grimoire could be found. The group decides to make a diversion there before heading to Marseille. Pursued by a horde of rats, they lose their mare, Provence, and are forced to continue on foot. They come across a group of children led by a disfigured giant wearing a pig mask, Augustin. Thinking she is being attacked, Amicia hurts him by throwing a vial of Devorantis at him. It turns out that this curious team is from the monastery and has decided to run away. They are forced to return to the monastery to treat Augustin.
- The monastery is run with an iron fist by Marcel, a ruthless monk who shamelessly mistreats the sick and refugees in his care. We also meet André, a good-natured cook. Augustin is a man with a mysterious past who was disfigured before being taken in by the monks. Marcel forces him to wear a mask at all times. Olivio, on the other hand, died a short time before.
- The group splits up after their arrival to find the grimoire. Lucas meets André and helps him in the kitchen. Hugo joins a group of orphans, which leads to some very touching scenes. Amicia, in her search, realises that Marcel has destroyed many of Olivio's books and that there are corpses in the cellars eaten by rats...
- The group of children decide to look for Augustin, who has been imprisoned by Marcel. During their journey, Hugo has a seizure and triggers an earthquake that causes him to fall into the monk's cell. The monk makes a strong impression on Hugo and comforts him. He tells him that his nightmares are probably due to melancholy and fear. Finally, he will teach him that no matter how dark it gets, there is always light and he will get through it.
- The troop of flagellants besieges the monastery and demands that Hugo be handed over to them. Marcel refuses to give him to them and orders the group to be imprisoned. Hugo's seizure triggers an invasion of rats that attack everything. When André realises that Marcel has left the plague patients to die, he pushes him into the cellars and leaves him to die. He will evacuate the monastery and leave with Hugo's friends. Augustine sacrifices himself to allow Amicia, Hugo, Lucas and Beatrice to escape. They return to their journey confident that Hugo has succeeded in defeating the Contra Impera.

Requiem
- Sophia is approaching her thirties. Her father was a charismatic smuggler who tried to join the high nobility by seducing her mother. She grew up listening to his tales of adventure and wanted a life of freedom. When her family learned of her father's past, he was cast out and she was put in a convent. After several attempts to escape, she killed two men who were under the influence of alcohol. Wanted, she has been living an outlaw's life ever since. The moment when she saves Amicia by killing the pirate captain is very important because she had sworn not to kill again.
- The Count of Arles is fifty years old and already has children from a first marriage. He falls in love with Emilie after they meet at a reception. He imposes his yoke on Provence through brutality and believes that only the army can enforce order. After learning that Emilie cannot have children, he discovers the myth of the Child of the Embers to his wife. They decide to settle in La Cuna.
- Emilie is 26 years old. She had a traumatic childhood because of her manipulative and violent father. To ease her depression, she immerses herself in myths and legends. She is convinced that having a child will solve her problems but learns that she is sterile after marrying the Count and becomes ill. After discovering the myth of the Child of the Embers, she falls in love with the place and feels the presence of the child everywhere. She is convinced that she is the mother of the Child.
- Arnaud's family name is Malpart. He is 40 years old. He was a knight before becoming a mercenary. Following the count's betrayal, he lost his son who was a young sentry in charge of collecting information for him. He is looking for Hugo to get his revenge.
- The little inscription written on the Derune's medalion, "Compositum De Compositis Sigillumveratis" can be roughly translated as " composite of sealed composites ".

Please, feel free to comment !

My sources :
- A Plague Tale : Tenebris
- The Heart of A Plague Tale
- A Plague Tale Requiem Artbook