Here is the full list of all 15 The Mooseman achievements.
Return to the gates of the Lower World
Collect all artefacts
Survive the chase by Voipel
Enter the world of gods and ancestors
Outsmart Yoma the Witch
Enter the world of men
Solve the riddle of Vorsa's sanctuary
Escape the river god
Find all the artefacts among Mangonth's tusks
Fight and defeat the spider of the Lower World
Grab a piece of shimmering sun
Solve the ice crystal riddle in the underworld
We too have a cow level
Enter the world of the dead and ancient spirits
Outsmart the guardian of the Lower World