Demon

Origin
Demons are a race of beings created and born from magic and emotion. All family lines of demons have spawned in one of two ways, either by being summoned as a familiar or spawning from chaotic influence in an area rich in magic. The first demons appear very shortly after the year 0 AM via the rising magical energies and the panick of the people of that era as the world changed around them at an unimaginable pace. These first demons were incredibly feral and terrorized the world until the rise of the Godly Order of Humanity's Light (GOHL) led to a crusade against demons, wiping most of them out. The first Wizards' Guilds began their studies of demons soon after and learned to create their own as familiars. A fact that has led to conflict between GOHL and virtually every Wizards' Guild that persists to this day.

General Information
In their weakest forms, demons are not truly sentient. They take form and behave based on three primary factors: the element they are born from, the purpose they were created for, and the emotional energies utilized in their creation. They however have near unlimited potential for growth and the longer they are allowed to live, the more sentient they become. This sentience can be dangerous if the demon was poorly treated or has been living feral for a long time but overall their behavior can rapidly become indistinguishable from a human's.
Generations beyond the first share many of the traits of the first generation with one additional trait: they hold a piece of the soul of their parents. Demons cannot reproduce merely by the means we might be used to. It can be used as a component of reproduction but it is not the primary element. Instead to reproduce a demon must rip out a piece of their own soul, essentially tearing out a part of their own memories and personality, to fuse with another soul or be reshaped into a whole new soul. The fusing with another soul is the more common method due to it requiring less of the original soul to perform and removes this being a requirement for future generations.
Given that demons are in part born from emotion, those born from demons tend to be a bit more emotionally volatile than most. A large part of this is that as demons are born from emotion they also feed on it in some form. If a young demon isn't surrounded by the emotion they are born from, they may instinctually seek to cause this emotional state in others or themselves to feed off of.
Demons are also unique in that their magical element is a dominant trait in their children. Short of sealing away the demonic part of the soul or a more powerful parent, a demon's children will always have access to the same kind of magic as their demon parent. Given that it's normal for parents to pass on some aspect of their magic to their children, this isn't usually a notable fact unless an individual has multiple children that share identical traits.

Physiology
As state previously, pure demons are beings of emotion and magic given physical form. This form is usually determined by the imagination of the creator or the collective imaginations of lingering thoughts and emotions of the place that spawned them. Most are given a simple form to start out with and grow from which adapts rapidly to their purpose or life. It is ultimately fluid however with the will of the demon ultimately determining the final shape their body takes. There is always one aspect that is the same however. Every pure demon has a "core". A place in their body that regulates the flow of magic between body and soul. The shape of this core is varied however. Some demons think of it as their heart. Others think of it as their brain. Others have recognized it to be whatever they wish it to be and are able to freely manipulate its final placement in their body. This is the only truly vital part of their body that they cannot survive without. However given that their form is determined by their will and imagination, their bodies can often end up with unnecessary weaknesses. A demon who studies human biology for example may imagine themselves as a human form and give themselves human organs that function mostly the same as that of a normal human, whether consciously or subconsciously. While another may simply take a form but treat it as simply a shell to contain their being. There are in fact advantages and disadvantages to both approaches as the purely magic-centric approach may make one more vulnerable to phenomenon that more strongly affect magic while the physical approach naturally results in the strengths weaknesses of the physical body.
Those with only partial demonic heritage physically have no inherent differences from the physical being they are born from. However the demonic influence will bleed into their magic in some form, even if they somehow avoid having the same affinity type. The most common example is their magical abilities having an innate aura of the emotional element of the demon they came from. It is usually subtle such as fire magic that might create a slight sense of comfort or passion versus that same kind of magic creating a sense of fear. One with control over this emotional element, both part demon and full demon, can use this emotion to fuel their magic.

Reputation
In most circumstances those who are part demon are treated as any other individual would be, essentially just another race in a world full of a multitude of many different beings. However GOHL and other sects, particularly in human-centric nations, see magically created beings as an affront to nature or their gods. This makes anyone with any sort of demonic heritage an obvious target due to their magical origins.
Full blood demons however are a different circumstance entirely. While they have just as much capability to be just like anyone else, there are some traits that have lead to a particularly negative reputation. Feral demons tend to be animalistic and destructive even after gaining sentience due to their animalistic lifestyles, so areas that spawn demons regularly tend to have a greater level of distrust for those with demon heritage. There is also the splitting of their souls for reproduction. This action is traumatic at even the best of circumstances and quite literally removes a piece of who they are. This can mean that a demon who is friendly one day can suddenly become hostile if that removed piece of themselves happens to be something that defined their friendly nature. A demon who passes on their sense of loyalty can turn traitor overnight. Worse, even if a demon is more cautious about what and how much of themselves they remove, they can descend into madness due to their fragmented being.
There are also rumors that full blooded demons, with their ability to manipulate their own souls, could potentially learn to manipulate those of others. In fact, demons that are capable of draining life force or magic are often mistaken for doing just this and are almost universally hated.

Weaknesses
Faith-based Magic - Faith magic is fueled by sincerely held belief. Given that certain faiths can see certain aspects of demons as inherently evil, this means that it is easier for faith magic to create spells well-suited harming demons.
Anti-magic Weaponry - Given the magical nature of demons, weaponry designed specifically to disrupt magic strikes at their very being directly. The less of a physical form a demon has, the more vulnerable to this they are.
Isolation - Isolating a demon cuts them off from the emotions they feed on, eventually starving them.