Like all societies, be they ancient or modern, the citizens of Polaia have a lower class, a sub-culture of slum-dwellers, an ‘avoid eye contact with them’ group. The Muwau are this on a formalized scale. For a basic understanding of just what that means, take a look at this article about the Dalit. The Muwau take up many of the same rolls of handling ‘unclean’ jobs (although they only handle the burial of their own dead and waylaid corpses that have no realistic chance of being buried properly).
That having been said, the concrete of how they came to be is quite different. Instead of being the ‘leftovers’ after the rest of the castes were made, Muwau are considered to be the byproduct of Iwau’s feeding. While it is true that Iwau is a god, the fact remains that he feeds upon souls. Whether they are aqcuired from the improperly buried or hunted down for sport, there are times that Iwau finds himself gorged upon to many lives. On nights like this, Iwau vomits, turning the moon red and unleashing a swarm of undigested spirits. These spirits then become reincarnated as Muwau. Being such incomplete souls is why they are scorned, and why they were named Muwau, or ‘dog vomit.’
It should be noted that not all Muwau were born into the cast proper. Should a man of any station commit a sufficient crime, like murder or crimes of a large enough scale, they can be demoted to a Muwau. This is religiously justified by acknowledging that not all the souls vomited up by Iwau neccessarily are reborn into a Muwau family. The end result is that most parents experience greater ostracizement and ridicule than had their child simply been executed or exiled.