Unlike Eldrich creatures which are said to originate from the darkness of space, Demons appear to come from an alternate plain or realm.
When a rift is formed between their realm and the physical realm Demons are able to slip through. Once through they tend to spend their time amusing themselves. (the realm they originate from must be very boring to them.) Depending on the type of demon this could be manifested as anything from a simple annoyance to something completely dangerous. Though they tend to act on their own impulses and ideals, at times they could act in the interest of both parties involved in a situation. Their interactions vary heavily though, and it's best not to risk interaction unless you know what you're dealing with. Demons can be very violent and unpredictable.
Demon entries to follow...