i have cleared out jorils cavern, and the whole of wyrms tooth, but how do i get to the lower levels of dorn, in chapter 6

Head out to the cave in the upper right of Wyrm's Tooth to the small canyon. From there you can go to the upper right to get back to Vale of Shadows, Hand, etc., but if you go through the cave on the right in this canyon, you'll get to Lower Dorn's Deep.