Demonic Transformation is a rare and powerful ability.
The first beings to become demons were the Gods/Angels who rebelled against Heaven. In the battle that ensued, the rebels were defeated and cast into the fires of Hell, from which they were reborn as demons.
Half-demons possess the ability transform into a demon at will. The gods refer to this as a forbidden power and publicly shun hybrids for it. The only known demon hybrid was Amira, an artificially created half-angel/half-demon.
Martinet was infamous for creating a concoction that could turn regular humans into demons. These demons were loyal to Martinet's cause and lacked their original kind hearts, disregarding former friends and allies upon transformation.