It's nowhere near the other puzzles in this column.
Map
River Obelisk
North #2
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It's made of negative space.
It's CHALLENGING to find the right angle.
Try looking from that one pillar puzzle.
Standing by the the pillar that opens the door to the Challenge, look up and to the left. The skylight will form the shape.
Complete spoiler:
Roll over each black area below for hints.
Each new hint gives away more than the previous one.
It's inside the mountain.
In the biggest room.
By one of the exits.
... and explored inside pretty thoroughly.
Prerequisites for solving:
You have to have gotten into the mountain.
Hints:
Intro
This one is hardest to find when you are looking for it.
Show hints
THE WITNESS - ENVIRONMENTAL PUZZLE HINT GUIDE