"Burninating the countryside, burninating the peasants, burninating all the people" - Trogdor the Burninator, by Strong Bad.
The demons are big burning creatures, basically. They're not evil, but they certainly look it. They sort of spawn out of the volcano lying under the Obsidian Mountain Range.
A demon looks like a human except for a few small differences, noticable only by experts. Demons are seven or eight feet tall, on average, and on fire. They can stop the fire from burning stuff, but it's always visible. They tend to have horns in unique patterns on their bodies, and usually on their heads. From the waist down, they have sort of flame-skirts of varying lengths, largely because I found it easier to draw them like that. Their skin is can be any shade of red, orange, or grey. The tougher ones tend to have markings on their faces and sometimes bodies, but it's not a requirement. A few also have obsidian embedded in them. The only clothing demons ever wear is a flame-resistant belt. Demons do not have mouths, but can communicate nonetheless, through a form of telepathy.
In terms of power, they're brutal, ordinary citizens have no chance against them. Even experienced warriors, unless properly prepared, go down like nothing.
The first Demon emerged way back, far enough back that you don't have to worry about exactly when it was. Before the formation of the Haram Empire, at any rate.
At first, it was simply a sort of moving flame, with no true form. Eventually, though, this first demon encountered humans. It molded its form to copy theirs, and it became known as the Firstborn, and learned the customs and language of the people. Over time, more demons began to emerge. The Firstborn, once again borrowing a human practice, took the title of Lord, and taught the others what he knew. Thus originated the main habits of the demons- taking on a humanoid form, using a form of human speech, wearing belts. The demons persisted in a state of relative peace for quite some time.
A while ago- hooray for our brilliant story continuity, I'm still trying to fit it together properly- they all got sealed in the volcano for over a hundred years. Long story completely omitted, they came out weakened by Avran's Curse. They're still bloody murder on their home turf, though.
The demons are, at heart, a warlike race. This is not due to any particular natural violence, but is a fruit of their lifestyle. To live, demons do not need to eat. As long as there is oxygen around them, they can keep burning, and stay alive. They need not worry about survival or death, since they are effectively immortal unless killed. Thus, they are constrained to philosophical pursuits or fighting to give meaning to their lives. Before they were bound by Avran's Curse, most demons roamed the surrounding lands, fighting for whatever reason they preferred- helping those in need, putting down arrogant little mages, or simply for the hell of it. Due to their collective origin in the volcano, demons do not normally fight seriously with one another under any circumstances. There are a few cases where this does not hold true, but they are few and far between.