There are two different Dark Elf watchful sentries.
There is one in the southeast room at ( -68, -10, -59 ) Copy/waypoint -68, -10, -59, level 32^^^, killing it unlocks the door at ( -64, -10, -82 ) Copy/waypoint -64, -10, -82
The other is at the end of the southern of the two west-leading hallways from The Inner Guard at ( 64, -10, -96 ) Copy/waypoint 64, -10, -96, behind a fake door. It's level 35^^^. Killing it unlocks the door at ( 4, -10, -91 ) Copy/waypoint 4, -10, -91