To operate the switch, you have to find (and carry) the Black Wolf Talisman, found in one of the side crypts. It sort of functions as a key. It's a rectangular gray stone with a pawprint of a wolf painted on it in white. It has magical properties as well, a good item to wear.
There are altogether 7 keys, counting both the talisman above and the Holy Symbol of Myrkul (which allows you to open the north door on the first level). Presumably you have the holy symbol, so the talisman is the only one you're missing.
Fortunately, this is the only dungeon where you have to scrounge for a lot of keys.
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