Mechanus and Modrons

Mechanus is the plane of pure law. Souls of people who sought rigid society become Modrons at the center of the plane. The Modrons are led by a mysterious entity known as Primus. Each Modron has a place, with the lowest answering to the one directly above it, and that one to the one above it, etc. Their purpose is to maintain the large clockwork cogs that make up the plane. Why this is needed, only Primus knows. Any influence from outside Mechanus brings chaos and is met with fear, revulsion, and in extreme cases, violence.

Acheron

Acheron is the third level of the Underspire. It is made up of countless massive cubes that constantly crash into each other. The Orcs and Goblinoids that inhabit the level constantly war in the name of their warmongering gods. Skirmishes erupt often when the cubes collides. The fighting only stops there when there are no more of the opposing forces. The one place explorers are welcome is Gulmag, a fortress run by Hobgoblins that allow visitors rest in exchange for conscription or tactical advantage over the Orcs, such as battle plans or magical weaponry.