This achievement is unlocked when you find this artifact:
#40 Man and Moosemen - 26:49
This one is a bit tricky to explain. Keep your light on through the entire puzzle.
After escaping the river, you'll find three totems in a forest. Turn on your mask and light and the eyes of the left-most totem will glow. Stand in front of this totem to start the first sequence. When the sequence is done, stand in front of the first totem and turn your mask on and off to make the eyes glow. Repeat this with the rest of the totems in the sequence.
The sequences are (from left to right):
Complete all three sequences to reveal the artifact and unlock the achievement.