Skull Pass is a giant skull-shaped passageway through the back roads of Mistral City, leading to Verigan's Hold. Xephos and Honeydew spot Israphel entering the cave during "Shadow of Israphel" Part 11: The Crumbling Ruin, but he had already gone through by Part 12: Beyond Skull Pass came (due them wasting time on some Custom Maps). It is unknown whether the giant skull is a natural formation or was carved there as a tourist attraction by the Mistral City tourism council.
Instead of finding a sprawling maze, they found a water ride leading to Mr Banjo's Carnivale Del Banjo. By Part 17: The Fragmented Map, the water ride had been taken down as the Carnivale was moving out of town.
Interestingly, a similar skull, though with golden teeth, was seen in Grimjaw's Treasure Hold.