Magic & Magi:
Magic is an art form learnable only to those who have an attunement to mana. While the advanced spells may only be learned by those that possess a strong control over mana; common people may also learn rather on their own or from a teacher some lesser forms of magic. A Magi is typically the term that is tied to any person that has learned any form of magic. Lesser Magi are common people that have learned to wield the easier to learn spells, one of the most common one is that of creating a small light source. Elite Magi are those that have been accepted into their nation's forces.
For one to become an Elite Magi, they must present themselves to a grand master of the art to showcase their skill and control. Only if they pass may they then be presented to their nations King to again showcase themselves. If the King approves and find's a use for the skills shown, then, and only then can that person enter into the training needed to become an Elite Magi.
Elite Magi are required to learn a wide range of magic in order to work alongside their nation's soldiers in safe guarding the people and preserving order. Elite Magi are also often joined with a beast partner that will aid them. These beasts are 'linked' to the Magi by having a small portion of the Magi's spiritual energy infused with in them. The beasts roles vary based on the Magi's needs, some may serve as lookouts while others may aid in defense or patrol. Being linked, the Magi may also barrow control over their beast partner to perform various tasks, such as seeing things though their eyes.
Demons:
'Demons' as they are known as, are not native to the world. They only started to appear with the rediscovery of darker magic that had been used once in decades past then sealed away for 'unknown reasons'. This darker magic is that of summoning. Magic that allowed Magi to open rifts from our world to a netherworld and call forth its inhabitance, 'demons' to do the magi's bidding.
The name 'demon' is perhaps a misleading one, as there are no signs that they are an 'evil' or 'demonic' entity. The term 'demon' is only referenced due to their ability to 'possess' per say a person soul, and that they are summoned from a Netherworld. Nether Demons also don't have a specific weakness to one elemental alignment over another (such as holy) an earth based spell can be just as effective against them as a holy spell.
In their original form, Demons appear to be purely energy-based creatures, appearing as a red glowing mist of various sizes and forms that remain so unless they merge onto the soul of a living being. In their nether form, physical attacks cannot harm them, although they are still vulnerable to magical assaults. If the Demon however has become merged with a human or animal host, then they too gain the same vulnerabilities they do, though some may still show a higher resistance.
Demon Bound:
An individual who has had their soul merged with a nether demon. Those that become bound will be 'branded' by a nether mark somewhere on their person. In some cases one or both eyes may also turn red.
A person who is bound will gain a large portion of the demons strength and power, along with an extended life span (supposed immortality). It is the thought of power and eternal life that draws many to willingly allow a demon the merged with their soul in exchange for their 'gifts'.
While bound the demon may influence the person's actions or in a worse case, take over the person fully. There are no sign's to tell witch of these are the case, and it is common belief that if you are bound, you are under full control of the demon.
Not so common knowledge:
Most people believe that a demon may try and possess an unlucky soul at any given opportunity, leading some to go to great lengths to prevent it using wards and other means to keep them away. However studies show that possession of an unwilling soul is very rare.
That may be due to man's frailness. If a demon tried to possess an unwilling soul, the struggle against it may be too much to bare, or the demons power may be too overwhelming, leading to the person's death. This may also lead to the demons own death, if not severely weakening it if they had become only partly merged. It is for that fact that demons seem to seek out those willing to summon them and allow the demon to merge with their soul willingly in exchange for the demons power and what many believe is immortality.
Animals are far easier to merge with then man, and due to their simple minds they are easier to fully control. However beasts are unable to wield magic so the demons would be limited to only the animals strength merged with their own.
Theories:
While a demons intelligence is unclear, it has been seen some to be more so then others. Some perhaps being on par with that of a human, others less so. This is seen in how nether demons seem to prefer a willing human host rather than say a beast. Along with how they appear to learn from their host's known skills and desires.
Some magi who have studied the nether demons more closely speculate that they may not be purposely turning the people they possess 'evil', but rather they are continuing their natural behavior from their world in ours. As if taking a wild animal out of its normal environment and placing it in another, possession may just be one way they have learned to adapt to our environment and learn our ways.
When a person becomes bound, it is believed that along with the demons power, that they also gain immortality, a 'forever life'. However that may not be the case, while it is known that those bound have an extended life, it may very well just be the demons own life energy keeping them alive, and while no one really knows how long these demons may live, there have been some seen to have 'passed away' possibly due to age, or even ailment.