Unseen Revenant in level 9, Nessus:

A short, ground-level hallway runs horizontally along the
middle of the north edge of the map.  Another, vertical
hallway crosses OVER the horizontal hallway, and so forms a
plus-shaped intersection.  The south end of this
intersection runs through a false, intangible wall, and
leads into the large building in the middle of the map.
The north end of the intersection is a dead end, a raised
cul-de-sac, where a Revenant guards a few goodies.

Note that the Revenant can see through the false wall into
the building, and its projectiles can pass through the wall
unhindered.