Black Fingers of Everlight

where slumdogs save the day and help a mighty champion to atone

From the introduction:

In hopeless slums of metropolitan Sothis, the capital of desert country Osirion, a cult of Norgorber starts operating in a disguise of the sun goddess Sarenrae. Unbeknown to its subjects, the Mercy House of Reputable Dawnflower sends beggars and other unfortunate people to their doom, to be sold as slaves or to be used as test subjects for new poisons. Outside, the Mercy keeps up a facade of the Healing Goddess, doing minor healing work and distributing fresh drinking water. Word of the Mercy spreads fast in the slums, and their activities become widely accepted.