With a simple key bought from Sly you open the stone door near the East side of Dirtmouth. As you open a bug emerges from the darkness.

Welcome, small intruder. I've been sound asleep in here for...some time? Ah. Yes. Yes. Well now you've gone and woken me up. It's no matter, I suppose. I might even be able to help you. I am Jiji, and if you have found your way into my chamber you must need my help. I can return regrets back to you. Though, it doesn't look like you have any currently.