Demon (lore)

When the Witch of Izalith tried to use her powers to recreate the First Flame, the primordial source of life and diversity, she only sparked an uncontrollable surge of corrupted energy known as the Flame of Chaos. As a twisted imitation of the First Flame, the Flame of Chaos could not create anything but only warp existing beings, thus perverting all life in its wake: starting from the Witch of Izalith and her children, the Flame of Chaos transformed its victims into monsters that exhibited various traits of different creatures (like the Capra Demon, almost humanoid aside from its caprine skull and its four burning eyes) or unrelated species (the Witch of Izalith herself became the Bed of Chaos, an entity composed by a skeletal headless torso and an insectoid "core" and both were made of dessiccated brambles) and even imparting a semblance of life to inanimate matter (the Titanite Demon).

Dark Souls
Most demons in Dark Soul have demonstrated to be intelligent enough to craft their own equipment using archtree wood, titanite and demon bones, and none of them ever used weapons forged by other races. Despite their blasphemous birth and hideous appearance, the Demons had some sort of alliance with the Gods themselves. It is not known if there was a real agreement sealed due to the common hatred towards the Undeads (both Demons and Gods are born from fire, even if in different ways, while humans are closer to the Abyss through the Dark Soul itself) or if it was just a truce in the name of the Witch of Izalith (who, prior to become the Bed of Chaos, was one of the Lords alongside Gwyn and Nito), but it is possible to see this accord in various locations: not only the Undead Asylum is guarded by the Asylum Demon and a greater Taurus Demon but even Anor Londo -the city of the Gods- is home for the Batwing Demons (creatures tasked with the trasportation of the Chosen Undead between Sen's Fortress and Anor Londo) and for the Crow Demons (trapped within the Painted World of Ariamis alongside what was linked to their patron, the goddess Velka).

Dark Souls II
It is known that the Flame of Chaos, called The Old Chaos, birthed horrible beings that were battled by the Ivory King and his Knights of Eleum Loyce. Even if none of them returned alive, the utter absence of any of the Fire Ages' Demon could mean that the King's army menaged to slew all of them before soccumbing to the foul Chaos. The being called "Demons" encountered by the Bearer of the Curse did not have an evident connection to Chaos, instead they came in existence when powerful souls begin to fester with corruption: the Covetous Demon was once a man who started to gorge himself after Mytha broke his heart, the Demon of Song was a monster which craved the taste of human flesh and the Smelter Demon was formed when an iron construct engulfed the Iron King's soul. Two possible exceptions are the Iron King (who became a demonic monster when something below his domain, amidst the "fire that rose from earth", possessed him) and Lord Aldia (through countless experiments he discovered an unspeakable truth, thus becaming an entity made of fire and dried branches similar to the Bed of Chaos).