Sorry, Tuor, I cannot agree with you. The key you are speaking of is called "Sewer Key" and can be found on Tazok's body in WindspearHills dungeon. This key is really precious as it grants access to that location in the sewers under Temple district of Athkatla - exactly as you've said
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As to Chapel key, it is not needed for anything. There are rumors that this key is actually part of some quest that has never been built into the game.
Just throw it away, Heapajunk
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Tanda,
with best regards from Guild of Assassins