Half-demons are people, like general Ratava, who have been infected with a demon-virus. Half-demons will get inner demons, which will cause immense amounts of pain, and after a while, finally be able to switch back and forth between their normal and demon forms. Half-demons can decide to become fully demons, to get rid of the pain.
There is an other type of half-demons as well, whose parents are a demon and another higher form of life. Only one of such is known so far, Dracor Scaleflame, whose mother is the demon Anklya and his father is an unknown dragon, even though this union is strange.