Look closer at NE corner of the Underdark map. There is an entrance to the DrowCity there, right? Investigate this area more carefully - not far from DrowEntrance there is a way leading to another location - Adalon's lair.
Basically, just search NE corner of the map.
Hope this helps.
And -
welcome to the forum, JimJ!
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Tanda,
with best regards from Guild of Assassins