Demonic Transformation is a rare and powerful ability.
History
The first beings to become demons were 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.
These demons became one of the three major races of Mistarcia and began co-existing with humans and gods, residing mainly in Cocytus.
Hybrids
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.
Potions
Martinet knew how to create a potion or drug that turned regular humans into demons. These demons were loyal to Martinet's cause and lacked their original kind hearts, disregarding former friends and alliances upon transformation.
Martinet used this potion to turn Jeanne d'Arc and Favaro Leone into demons. Both of them were cured using an antidote concocted by Rita.