The Cairn Gods
Nadrow has many religions within it's rather vague borders. They worship tetrads and spirits alike, but the religion with the most following is known as the Cairn Gods. Originally a drow religion, it has grown and finds particular favor with nadruen elves.
At it's core, worship of the Cairn gods is about overcoming the evils that cloud your path, living your life in security of body and soul. The story goes that the gods were the leaders of the drow exodus from the underdark into nadrow thousands of years ago, with each god representing the counter to an "evil" spirit that would otherwise have kept the drow trapped in lives they wished to leave.
Notably, nadruens distinguish between two types of darkness, and so have two gods that defend against their relevant dangers. Mon is the name of the darkness within civilisations, of people who wish others ill and of abandoned structures. Krid is wild darkness, of beasts and monsters and the uncaring world that could kill you as easily as an elephant steps on an ant.
Cosmological Views
Worship of the cairn gods does not require a specific understanding of cosmology, but in general it's followers believe in tetrad gods, and usually in Skormir's Children and the cosmological implications of that.
The extent to which the spirits in Cairnite worship are considered real entities is a subject of much religious debate. Academic scholars of the religion claim there is no evidence for their existence, and argue that if they did exist they would surely be some otherplanar being. However, many normal carinites would swear that they had seen things, dreamt of things, tempting or threatening them with all of the evils.
The spirits (who do exist) are wise enough to go to only those that still fear them. Those who's connection to and knowledge of their gods is shaky enough for shadows to manifest and pull them back into the underdark ways.
