This page offers a guide on where to locate and how to complete Grime’s Graves, which is a Mystery located in East Anglia.
Travel through the tunnels
Grime’s Graves is a mine shaft located north of Britannia’s Watch and southwest of Brisleah Farm. When you enter down into the pit, you’ll need to head towards the tunnel to the northeast. For the majority of your time in the tunnels, you’ll want to be heading in a vaguely north direction. You’ll have to do a bit of parkouring across chasms, and there are a few secret rooms where you can loot different types of supplies.
After a short while it’ll become increasingly hard to see as the area is cursed. You’ll find a large room with a cursed symbol. For the sake of being able to see, you’ll want to cleanse the cursed area before moving on.
Once you’ve cleared the area, you’ll be able to spot a ladder heading up out of the cave. After this ladder, you’ll need to do a bit more parkouring to reach a large archway covered with a banner. Head through and you’ll find a small living area with a table on the backwall. Smash the pots on the table to reveal a secret treasure, the Mysterious Hilt Tablet.