The Nether energy. With the destruction energy inside his body Azief could easily refine the Nether energy.
It is an energy that flows through the realms of the living and the dead.
If he could form a Disk from that energy he could generate, conjure and manipulate nether, the very essence that flows through the realms of the living and dead
It also relates heavily with his Death Source. One could even say Nether energy is the source of powers from Death Source and other related energy source.
It is also the substance that makes up the underworld. One could think of it as a subterranean dimension where all souls go when they pass on.
Azief has been researching about souls since he created the Creation and Destruction seeds a long time ago.
Nether energy is the opposite of the Aether energy. It is similar but unlike it. It is like the Aether energy in that it is from a Plane that exists between the planes of both matter, and energy.