Glory to the Dominion! (Disgardium Book #9): LitRPG Series

Scyth, known in the Inferno as the tiefling Hakkar, decanus in the worst legion of them all, must gain an audience with Prince Belial to get his hands on the Coals of Hellflame.

His friends are also keeping busy: Crag, once more a member of the Awoken, heads to Terrastera to learn how to create a Rift to the Nether; Infect goes digging in the Lakharian Desert; Bomber speeds toward the underwater kingdom of the naga; Crawler strives to be the first in the world to build a magic tower up to five levels; Tissa and Irita focus on growing the clan; and new recruit Hiros must fight against his own demons.

In the meantime, the legates of the Destroying Plague gather an undead horde to march on Kharinza...