The Underpass is a pair of paths through a part of the Underdark (one 200-300 m down, the other 1500-2000 m down) that barely connects to the main body of the endless depths. None of the Vampire Kings ever enter The Underpass, as they all share an aversion to going underground for unknown reasons.
It has been demonstrated that The Albtraum can extend its influence outside of its realm into the Underpass, but it is not clear whether this is a unique feature of the Underpass or whether it is simply a coincidence that this is where The Party and Rygax were at the time. If the former, this may explain the Kings' unwillingness to travel there.
The Underpass is home to the acid rats, who live off local bugs.