hey in demon slayer i've already washed down the key into the drains, but took too long to get down and the key was gone when i got down there. Where do i go from there?

Try going back to Sir Prysin and talk to him again. It should reset. You might have to use a bucket of water again, so make sure your run energy is 100% before you do it and run down to the sewer afterwards.

i've tried talking with him and all he says is it's in the drain

You should be able to wash the key through again. See if you can use a water bucket on it. You might have to log out to allow the server to reset, either before or after talking to Sir Prysin.

It should be down there. Im pretty sure theres no time limit.
I think you have to right click the drain, etc.