Demon talk huh? :0
I have a few myself actually :0 (Ik this chat is basically dead but i like the topic so…)
The demons in my story were created by the main antagonist in a special experiment program, as a way to get back at her arch-enemy, who had beaten her twice and sealed away her powers. She created a set of 24 demon possessed items which she then used to slowly exterminate the Morphidae specie. (a morphidae is the unevolved wingless form of an angel.)
The demons are actually made using a Morphidae soul and infusing it in an object, which in turn becomes capable of harming a Morphidae, since its basically the same energy - you can say its like fighting fire with fire.
Now, i should probably explain the soul. Think of a peach - the pulp is the external magic layer and the seed is the core. The external magic layer is the magic quantity a Morphidae accumulates as they grow up; it dictates how much power the creature has and it also acts as a protective shell for the core of the soul.
The core is basically the life essence and the genetic pack of a Morphidae - it holds everything that makes the creature what they are. If the core is destroyed, the creature is erased from existence.
Out of all 24 demons, only 8 are sentient, as they were created using the Morphidae's entire soul - these are the so-called "prototype demons", who are a tad bit tricky to use, since they have a free will. The rest were created through a slightly different method, that allowed the separation of the core from the external magic layer; the daemons created using this method are a puppet like, with no mind of their own.
The true appearance of a Morphidae is similar to that of an eastern dragon, with masks on their faces. They usually are white and gentle looking, with soft features.
A daemon's default appearance resembles that of a Morphidae, except it's black, fire-like, more twisted and corrupted. Daemons do not have their masks on, so their faces look slightly deformed, with various horns sprouting here and there, their mouths look like they had been torn open and their eyes are just glowing orbs.
If their owner wishes, they can take on human form, but they still stand out a bit, as they all wear the same skin tight sleeveless shirts and are a tad bit taller than the average human.
This one right here is my main Daemon, Rin. He is a sword daemon, the last one to be created before the experiment program was terminated. He was originally created without a core, but after meeting his current owner, he somehow mysteriously received life - it is believed he is a "defect", since he was created in a hurry and some mistakes were made during the process. He came out very OP, but there was always something about him that unsettled everyone, therefore he was hidden away, so that no one may ever get their hands on him and abuse his powers.
The one above is his human form, while this other one is his true/demonic form - he transforms into it whenever he is summoned from his sword.