The Nether Saga
In the final clash of light and darkness, good and evil, that heralded the end of the world, a new race was born: the Nethers.
The sheer concentration of opposing elements and forces in one place created a rift between heaven and the underworld, fueled by a new form of energy called Chaos Energy, which can only be created artifically by an extremely difficult spell, which requires the souls of a hero, and a Destroyer. Because the spell also requires the magician to be perfectly neutral, and stealing the souls of the dead or living is a serious offense, it is impossible to perform unless both souls were volunteers. However, the gods wanted another chance to create another world, so they in fact performed the spell with the dead soldier's souls, creating the Netherworld.
This phase in the D&D game mainly consists of adventures in the Netherworld, except with all the races replaced with their nether lookalikes, and having been granted the ability to manipulate Chaos Energy. FYI, Chaos Energy looks like red lightning.