We're talking EPL of 11 or so, but the ordinaries should be walkovers. The bosses and odd monsters, however...
It's an island isolated by several factors: currents sweep away from it, requiring rowing to get there; there's nothing profitable on the island as it's just a rocky place; and the requisite reefs. Basically, it's convenient to many different urban areas, while still serving as a nice secluded place away from the authorities.
Iron Golems with limbless, eyeless, sensory-organ-less bodies inside powering them. Probably people who aren't even part of the religion, just forced to live endlessly inside of the golem, their life-essence powering it.
Succubi who are pro-"pain over pleasure." Maybe a small goggle of them, used for Joymaking and other naughty aspects of the religion only afforded to the higher ups.
You know the guys from Hellraiser? I always imagine those monsters as hardcore Zon-Kuthonites.
Creatures who love to be hurt and to hurt in return. Really strange, screwed up stuff. Animals with parts removed. People with parts removed. Creatures with apparatuses forcing terrible things to happen. I can already see the trolls without legs hanging from the chains in the ceiling, assaulting the party by pulleying themselves up and down.