MM

MM's, or Messangers of Magic, are an alien life form. They take the form of various earthen creatures, with the exception of white skin, a red circle on their backs, red beady eyes and two golden rings. Whenever an MM's body is destroyed, it can create another one immediately in a 100 kilometer radius. They have a unique abilty- they can grant wishes. However, as expected, every wish has consequences. 
MM's require a service in exchange for their wish- to become a Siqril, a magical warrior that fights against witches. You receive elemental magic and a magical set of clothes you can summon at will. The MM's work with the cleric hunters in order to hunt the coven of the black sun, even with their questionable (A.K.A none existant) morals. Every Siqril receives a soul gem upon the completion of the contract between them and the MM. Their body then becomes an empty husk without their soul, which controls their body from the soul gem where it is safely stored. This way any damage to the body isn't fatal, as long as the Siqril has enough magic left to heal themselves. Their bodies become zombies controlled by their souls, and they potentially can even block out the sensation of pain, although that also reduces their reaction time. 
In addition, whenever a Siqril gives into despair, they transform into a witch themselves. The cleric hunter's way of dealing with this is killing the Siqril before they finish transforming, which puts them at odds with the MM's. This happens because of the MM's ultimate goal- to harvest emotional energy. They are a race born without emotions, which posed no probem for them until they found about "Universal Entropy". According to this concept, during each convertion of energy a little bit of the total energy is lots. Little by little, the universe is running out of energy. To counteract this, the MM's discovered a way to create a massive amount of energy from emotions- which they cannot produce themselves. So they searched the universe for a world with more emotional energy- when they found Soderhalm.
Humanity and it's mortals were brimming with emotions, and through the lure of the wish they could create a contract that collects their emotional energy. They also found out that despair, especially that which is created when becoming a witch is the most potent emotion, so that is why they made the Siqril transform into witches.