The Under Gods are the lowest tier of gods. They are probably best described more as superhuman, however they are either descended from or created by greater gods than themselves. The vast majority of these gods live amongst the mortals, however some may form communities of their own. Besides their increased power, the primary thing that will set them apart is their knowledge. They will most likely have a much greater understanding of the cosmos and cosmic order than the majority of mortals, and the mortals they do know about it almost certainly learned about it from the gods, probably The Under Gods.
Not all words necessarily have Under Gods. Many likely won't. They will, however, be a useful tool for having gods who are much more involved in history and stories, without being too overpowered. Many may even be the main characters. They are probably often regarded as historical heroes or villains due to their elevated abilities probably drawing them into prominent roles throughout history.