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Introduction
A Devil's Favor is an antique 1930s Royal typewriter. It has the ability to materialize in a space adjacent to a seemingly randomly selected person. On it's materialization, the target will detect the sound of someone typing on an old typewriter.
Experience of the typewriter's audibility will vary according to the reaction of the person. If that person begins to immediately search for the source of the sound, they will avoid a deeper state of obsession that strikes those that ignore it for any reason. For the latter, the typing will eventually increase in sound until it's deafening (though no one else can hear it). The obsesser will return to the original site of the sound, ignoring all other thoughts and personal necessity, until the typewriter is found.
On finding the typewriter, it will hold a single sheet of paper with text offering to grant a favor. If the reader replies by typing a message back, a dialogue will ensue with an unknown sentience, until the reader accepts or declines the offer based on the written conversation.
Purpose
The purpose of the typewriter is to establish two way communication with a person and one of 72 demonic members of the Ars Goetia. The exact entity that the person is contacting will vary depending on the type of wish a person has internalized. So a person is matched up with whatever demon that is capable of offering a favor with the highest chance of acceptance (the person can choose to decline the offer and close the connection coming from the typewriter once a favor has been discussed, though it's a rarity).
The reason this device came into being was, on the advent
of technology, the Ars Goetia saw it as an opportunity to establish more links to humans via drawing them toward tools of communication and establishing a connection. The spontaneity of its sound and appearance works to lure in the vulnerable (those with a wish), as well as reduce the chances of its target casting wards, banishing, grounding themselves, etc., upon hearing the sound and obsession in finding the source has begun.
So what's the catch?
A wish can always be granted on some level, but the "debt" isn't really along the lines of something similar to a monkey's paw. More like a butterfly effect. So the grantee isn't even always aware of the effects of the wish being granted (though on occasion, they are). Extent of the effects vary between a random someone having an unusual string of bad luck to something more drastic (the rise of a serial killer, civil uprising, etc.).
As for the catch: time isn't really a linear thing for a demon. Their goal here is to strike a deal. If the target asks what the catch is, the demon will say that the debt has already been paid. After all, if the target accepts the deal (and is informed of consequences for more outlandish wishes), any debt will always be repaid one way or another, even if it's not from the person that made the wish.
For wishes that start treading into supernatural territory (super strength, mind control, telekinesis, replenishing wealth, etc.), the subject will be warned that the structure of however the wish is delivered can become overtaxed. Wishes for super strength will degrade the subject's body. Telekinesis can be granted, but will be extremely taxing on the central nervous system. If the wisher changes their mind on being warned, they can negotiate via the typewriter for something similar. So continuing with the super strength example, becoming a world famous athlete instead.
