If you're talking about during the quest Demon Slayer, then what you do is use a bucket full of water with the drain, causing the key to dislodge and go into the sewers. Then go near the Church, and there should be a manhole. go down into the sewers, there should be two hallways. go through the hallway to the left, then turn left when you get to the water, and behind a broken wall, with (i believe) skulls piled up, there the key will be :) hope that helps!!!
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bucket of water
Assuming that you used the bucket of water with the drain, the key should be here in the mud http://img502.imageshack.us/img502/6241/varrocksewers.jpg
just head down, into the sewer, now just follow the hallway north. Go to your left, and you will see a little glint where the key is, if you already don't see it.
Buy it, get it from the Hill giants.
use a rope on it and talk to that guy with you in the sewer