>>10742183Devil is typically a proper noun and refers to a given culture's chief figure or personification of evil. The term comes from the abrahamic religions and just gets applied to other cultures which also happen to have a singular 'evil' deity to identify. The Devil is Satan. Its actual characterization is loosely based on Angra Mainyuu, a zoroastrian deity (which you probably recognize from FSN) that influenced judaism during a period where they lived with that culture.
"Demons" are a general term for malevolent supernatural beings typically associated with the onset of disease, mental illness, natural disaster and so on in various cultures. The term once again comes from abrahamic religions, where they're specifically fallen angels that joined the devil in rebellion against heaven and now serve him in damnation, though the specifics of demonology is typically borrowed from other religions so demons often differ quite greatly in aesthetic, motives and behaviour from the figure they ostensibly serve. The earliest example of an analogous concept is the children of Tiamat, malevolent gods in sumerian mythology born by the creator deity to combat her original children, the sumerian gods, who rebelled against her.
There's some muddling because we often describe parallel concepts from other cultures in abrahamic terms, but the roots of the terms are straightforward and abrahamic. There is one, singular devil and demons are his servants and representatives. The devil himself is concerned specifically with the temptation of humankind to sin, while demons carry out that will in all sorts of strange and esoteric ways because of their disparate and eclectic cultural roots
