Here's the excerpt from MaG's walkthrough:
Go back to the Underworld. Take the Greek letter paper - rubbing of the plaque beside the Cerberus close to the workshop.
- Go to Tartarus. Take a rubbing from the panel left of the door to the Fire River.
- Go through door and be in the Fire River room. Take a rubbing from the plaque left of the door.
So if you've already taken the Tartarus rubbing, you're practically next to the door, it seems?
