Les Séanciers
Though many wars have been waged across Elbid, the Dragon Wars were by far the deadliest conflict the continent has ever known. While it is difficult to be sure, at least twenty million - and perhaps as many as thirty-five million - perished as a result. Nearly everyone who survived the war lost someone, and the people of Elbid were seized by an unprecedented wave of grief and mourning in the years that followed.
In response to this collective sorrow, many turned to occult means in an attempt to contact the spirits of the departed. Though The Church had long forbidden such practices, deeming them blasphemous, countless people across Elbid chose to ignore the prohibition. Mediums and magicians arose who claimed to commune with the dead, each relying on either innate psychic talent or idiosyncratic magical technique. In Arvoringia, these practitioners became known as Les Séanciers - a title that would later spread throughout the continent and even to the island of Albion.
Voices from Beyond
When one of Les Séanciers sought to reach the afterlife, they would hold a special gathering - a séance - in which the participants attempted to open themselves to contact with Beyond. The Séancier assured those present that their particular gifts and training made the practice safer than it might otherwise seem: they would direct the communion toward a specific soul - always said to reside in Heaven as part of the Divine Choir - and shield the bereaved from the influence of other, malevolent entities that might seek to exploit the opportunity. In some cases, the medium would channel the spirit of the dead into their own body; at other times, they would direct the spirits to act upon the environment - knocking on walls, playing instruments, or even manifesting a body of ectoplasm that could be seen waving to its living kin.
Many found great comfort in these experiences, convinced that their dearly departed had indeed passed into the care of God in Heaven, and were not the prey or plaything of some eldritch power. The clergy - whether of the Old Church, the Church of Albion, or the sects of the Convocation of the North - universally denounced such manifestations as false, declaring it impossible for a soul within the Divine Choir to be so summoned or spoken with. Yet the people largely ignored these admonitions, some even speaking of a "special dispensation" granted by God in Heaven to a generation that had suffered so greatly.
Charlatans & False Prophets
Many of those who make up Les Séanciers are simple frauds - pretending to contact the dead in order to extract money from the living. When unmasked, they usually protest their innocence and misunderstanding, and more often than not their victims continue to defend them - preferring the comforting lie they have purchased to any revelation the debunkers might offer. Though many find such charlatans reprehensible, they are not truly dangerous.
Yet not all of Les Séanciers are confidence artists. Some sincerely attempt to open their minds to contact with Beyond, trusting in their skill, determination, and faith to protect them from the perils therein. These practitioners vary greatly in their ability to achieve what they attempt - it is no simple thing to reach past the bounds of the world - and many who make the effort simply fail. But those who succeed pose a genuine threat to all.
When they do establish a connection to the greater universe, they invite the forces that dwell there to invade their minds and souls. Each can list the protections and precautions they claim to take against such intrusion - some even insisting that their own minds are too strong to be preyed upon. Experts on the matter, both religious and secular, generally regard this as recklessness in the extreme - and in more than one case, a Séancier has been seized by a malevolent entity that tore their soul from their body, leaving only a lifeless husk behind.
This, of course, is the best possible outcome of an encounter with the eldritch predators from Beyond. What truly troubles those who study the movement is that it is impossible to know how many of the sincere Séanciers have already been infiltrated by such entities, and are now quietly assembling cults devoted to their new patrons. It is this possibility that has led many to call for Les Séanciers to be outlawed entirely - yet so far, while individuals are occasionally prosecuted for illicit practices, the movement as a whole remains protected by its wealthiest and most influential adherents.
The Office of Vigilance
In Arvoringia, the Archbishop of Dises has taken a direct approach to eradicating Les Séanciers, directing her inquisitors of the Office of Vigilance to charge practitioners with blasphemy and bring them before an ecclesiastical court. This practice is not officially condoned by the General Conclave, but - like many of Archbishop Ashcombe's actions - it has been allowed to proceed unhindered so long as it does not encroach upon territories outside Arvoringia.
Those seized for trial by the Office of Vigilance stand little chance of acquittal. The inquisitors typically produce a few witnesses from among their own ranks to testify to the offenses, and the bishops appointed by Archbishop Ashcombe are only too willing to pass sentence on their word. Those who throw themselves upon the mercy of the court may receive a severe penance - some decades of hard labor under the Church's supervision, perhaps - but more often the sentence is execution by burning: a punishment many had believed abandoned in modern times, yet revived under Archbishop Ashcombe's orders.

Dr Levi here: Are there ways to detect if someone has been taken over by a thing from beyond? And should someone from the University of Cadbury look into this?
Not reliably; the entities from Beyond vary so widely that no single test suffices. Best to burn them all and let God in Heaven sort them out.
Archbishop, what do you say to accusations that there would be no Les-Seanciers without the Dragon War and there would be no dragon war without the church wars from which your very position spawned?
I Agree. The Old Church is irredeemably corrupt, and some of my Colleagues in the Church of Albion have also grown lax. The Old Church strayed from the path of righteousness, which led to the Church Wars, and all that followed. If a firmer hand had held the Old Church in check and destroyed the corruption growing within it, we would not have the problems we do today. I cannot change the past. But I can show the way to the future. No mercy for the corrupt. Fire for the blasphemers. If we spare the wandering souls, they will multiply and all will suffer. That is the lesson of history - we cannot afford to be gentle.