From the introduction: The lower levels of the Dragon’s Delve are a complex structure, with magic passageways and hidden tunnels intersecting with the more obvious network of rooms and stairwells, not to mention the chambers holding two gods. The Tomb of Nibul is a side area, existing partly in normal space within the material plane, and partly in the extra-dimensional area of the wendways. It serves as a bridge from level 16 to level 18, and thus players may skip the tomb entirely if they wish.