Underrealm is a universe focused around the hellish realm of Underrealm, and the three creatures that live in it: Argus, Arnus and Acauldrus. There is a hierarchy (also known as the food chain by some) that each creature must adhere to, lest they suffer a fate worse than death, with underspawn being the highest and mortals being the lowest.
Had the whole theme on my mind while I was making this, yet I tried to make it as hellish and scary as possible. Fun fact; I imagined patting on Argus'/Acauldrus' belly while I was setting up those drums.