All this is still true. The following may be true also.
The Mudmen don't call themselves Mudmen, though they do not consider it especially insulting. They are the Tan, they have their own language and culture on the fringe of civilisation.
Their culture is based entirely around hiding from Tolhoth, god of forge fires, who is intent on eliminating them to ensure his victory against all other gods, as foretold by the Oracle of Pellan and explained previously. While covered in the mud of Loch Doldrum they are invisible to him.
Tolhoth only killed most of his followers. As result he only killed most of the gods.
The grey clay they use to cover themselves is the finest clay around. The Potters Guild at the foot of Holy Mountain have made a name and fortune for themselves on the stuff. The mudmen do not like this one bit.
"The flames treat the clay and turn it hard and strong."
The mudmen know they are the only humans left. all others are ghosts, vengeful and jealous that they did not escape the flames. This, combined with the Potters's continued exploitation of them has made them exceedingly hostile to outsiders. Or ghosts, more accurately.
They were once the Tan, who lived beyond the mountains and worshipped all manner of domestic gods. Most of these gods are now dead, victimes of Tolhoth's crusade, but while the mudmen live he cannot finish the job. The mudmen fight for your religious freedom.
One of the gods they consider to have survived the purge bears a remarkable resemblance to Vorn.
The Potters know exactly who and what the mudmen are and have suppressed the knowledge so that they can continue to rob them of their clay. If the church knew they were orthodox worshippers they could possibly be afforded some protection, in as much as any human would, from the depredations of the potters. But they aren't human, they're Mudmen.
The Burrow is what the potters call the Mudmen's city. Burrow is more of an accurate term than city, as it is literally a series of warrens cut into the mountain side and decorated in air dried clay embellishments. The actual name of the city is Great Strenk.
Mudmen are only without their masks when under the mountain. If caught without a mask they will fall into a catatonic coma and consider themselves to be dead. If other mudmen witness this they will agree, they are indeed dead and treated as any other ghost: determinedly ignored at best, attacked at worst.
The potters sponsor adventurers to go into the burrow, claiming it is a dungeon full of treasure. It is indeed full of treasure, and dungeon-like. The mudmen will not stand for anyone entering, they can't let outsiders see their faces.
They leave the burrow to hunt and gather clay for their disguises, returning there at night. If a mudman gets stuck outside they will bury themselves in the muddy shore with reed breathing straws. They cant risk their hats falling off while they sleep and having Tolhoth coming back and stomping them into dust.
Disturbed mudmen, covered in thick mud, pouncing from their partial burial. This is responsible for rumours of them being actually made of mud
Some potters are concerned that the mudmen are right, and tolhoth will destroy the gods is they succeed in driving the mudmen to extinction. They are a small minority, and still mostly content to let all their money console them.
The abuse of the mudman population by the potters is occasionally opposed by other civilised folk. However the potters are rich and maintain the only standing army for leagues around. As "guides" for those extracting the clay.
Ultimately, the mudmen just want to be left alone.