Demonic Plagues

Demons are known to infect humans both in fairy tales and in reality, but the Church prefers to keep a tight lid on their real life appearances. Nonetheless, there are some key features that are prevalent both in the real life appearances and in the fairy tales.

Demons are charlatans, traders of the soul and sadists; they will do anything necessary to further their own agenda and typically care little for human life. This does not mean there are no exceptions, rather it just so happens that the bunch of demons reaching out to the mortal realms are typically the ones with least consideration for others. The process of reaching out to these realms is tiring and dangerous and as such demons tend to favour targets of influence or strength, going after knights, veterans, nobility and mages more often than common peasants. The general process for a demonic infestation, also known as being subsumed by "the darkness", goes as follows:

1. Weakening: The target suffers either a direct physical injury that limits them and brings them pain or they suffer mentally over longer periods of time. Targets suffering from both or either before the other, i.e. someone losing their legs and having great grief over it, or someone who was already depressed suffering a new injury are the most vulnerable. This weakened mental state brings in the second stage.
2. Contact: The demon starts making contact with the target, whispering in their dreams, slowly dragging them towards the darkness and so on. This process can take anything from hours until months, the important thing is that the target cannot be too aware of what is happening or else their mind will block them out. After the target is "comfortable" with the dark whispers, typically observable as a person becoming cynical, nihilistic or highly introverted, the next phase can begin.
3. Bargaining: The demon starts offering deals to the target. The target may or may not become aware of the presence of the demon at this stage, but most will be too influenced to care any more. The deals offered vary greatly based on the demon and can range from removing ailment and pain, to restoring the legacy of a disfavoured house. Whether the demon is capable or not of the feat is questionable, but demons tend to not promise something they have no chance of accomplishing. If the target is in great pain, i.e. dying, this stage can nearly be a process of blackmail, where the demon increases the feelings of distress and pain until the target agrees. Once the target seems likely to accept the deal, the final stage is reached.
4. Completion: The target is given the promised deal from the demon and the demon is in return moved to the mortal realm, although the target may not always be aware of this part of the deal. Here, the demon might slowly consume the affected person, taking over their body if that was not already part of the deal, or guide them into performing dark deeds. The demon might also use their power for a myriad of other goals, but tend not to harm the target directly, understanding the value of future trades.