Demon
Demons were abomonations from the Abyssal Waste. They consumed souls to grow stronger and took many different shapes.[1]
Physiology
Demons took many shapes, and their shapes changed depending on how powerful they were. Scholars usually separated demons into nine categories, where demons in each category shared many phsyical attributes. Differences between demons could be in the form of extra eyes, teeth or limbs.[1]
Soul consumption
Demons consumed souls to become more powerful. It was said they consumed souls as other creatures consume food. Scholars delineated distinct power levels as categories, where categories 1 through 9 ranged from weakest to strongest.
While it only took one consumed soul for a demon to evolve from category 1 to category 2, it could take anywhere from one to thousands of souls to evolve from category 2 to category 3. There seemed to be no pattern for the number of souls needed to evolve from one category to the next.
The evolution from one category to the next was a process that varied in length. Sometimes it was almost instantaneous, other times it could take months or years.
Once a demon evolved to category 6, they started to develop a unique form, distinct from other demons of the same category. They also gained a sense of identity which persisted through further evolutions.
The most powerful demons, category 9 demons, were called demon lords.
Demons had the ability to expend some of their consumed souls to empower their attacks. If a demon was to expend all of their souls, they entered a state called lathe. While in lathe, demons were ravenous for souls and would attack any living thing in a mindless hunt for souls to consume.[1]
Soulsight
Demons had the ability to sense any nearby creature with a soul. They could do this even through objects or walls.[1]
Sociology
Demons were usually independent, but could in some instances make alliances. Such alliances only lasted until one of the demons turned on their allies, consuming them for their souls.
The only thing that can control a demon, is a more powerful demon.[1]