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The fear that the Fellsoul possesses can be any fear that an intelligent being may possess. Fell-Demons are created when the fear of a thing, action, or idea (henceforth dubbed as the entity of the fear) grows to the point of being able to mix with fire and ash in Mt Kazan to produce the Fell-demon. Thus, this fear is intrinsically linked to the Fellsoul.

Domains of fear are divided into four categories that are determined by how many intelligent beings fear that entity versus how inherently strong the entity is. The categories are FW (FEW fear it and the entity is WEAK), FS (Few fear it and the entity is Strong), MW (Many fear it and the entity is Weak), and MS (Many fear it and the entity is Strong). The DM will decide what category any one fear will fall into.

An example  of FW could be light. Relatively few intelligent creatures would fear light and light itself is not very strong.

An example of FS could be a black Lord Dragon, hidden in the the depths of the forsaken dark, that only the bravest of adventurers know of...and fear greatly

An example of MW could be spiders. Many simple folk fear these small critters, but most of them are as harmless as a fly.

An example of MS could come in the form of a vicious serial killer that has ravaged the land and cannot be stopped by mere men.

Despite the Fellwitch's best efforts to perfect the Fellsoul