There is a masked society which regularly meets in secret to discuss the Soul Arts and how to best harness souls. They embrace the Soul Industry, the trafficking of human and demon souls. Their power comes from the wealth they gain in their illicit dealings as well as their keen understanding of magic. An example of this magic power is the epee rapier each of them carry. These harmless, ceremonial blades have been imbued with an intense flame which easily scorches flesh on contact. In place of a shield they wield a dagger which is most adept at parrying the blows of their opponent.
Part of their wealth is used to hire mercenaries and clans of assassins who have been instructed to silence all who would oppose the Soul Industry or who are attempting to defeat the Old One. Should the Old One be defeated, the Soul Arts would once again be lost and with it their source of revenue. The highest ranked members of the society wear luxurious, black and gold robes and a gold mask. Mephistopheles is one of, if not the, supreme leader(s) of the masked society.
The Soul Industry has a particularly strong connection to the Tower of Latria as the corpse of one member is found in the dungeon (rogue's clothes & parrying dagger), the corpse of a few others were inside the demonic womb beneath the Old Monk's tower (epee rapier and gold mask), and one of their assassins, Yurt, was found by the player locked in a cage. This implies Latria was either the base of the masked society's operation or one of their meeting places. The base of the Old Monk's tower, where the womb drops, resembles a "sealed chamber" (one of the society's meeting locations) so it could be the Old Monk was a member of the Soul Industry and that is the reason he was banished from Latria.