Mazo (Demon World Miasma)

Mazo
As described in Chapter 10 of the Blood Lad manga, there is a miasma in the atmosphere of the demon world called Mazo. A demon's body converts Mazo (the miasma) into their Maryoku (the individual's magic), which they expend by using their Maryoku (offensively or however their particular Maryoku may be used). When a demon dies, their Maryoku is sent to the Human world, as a soul, which is reborn into a Human. When Humans die, their soul travels to Makai in the form of Mazo.