Here is the full list of all 18 Munin achievements.
Complete Ásgarð
Complete Svártalfheimr
Complete Ālfheimr
Complete Múspellsheimr
Complete Vanaheimr
Light up 4 Runes at the same time
Complete Jötunheimr
Watch the whole credits at normal speed from the Chapters Menu
Complete Nifleheimr
Complete Helgardhr
Complete Midgardr
Stand on top a rolling boulder for more than 3 seconds
Finish the last level in 45 seconds or less
Die in all the different ways possible
Collect 999 Feathers
Finish 9 levels without jumping
Free 9 Orb spirits
Collect 9 Feathers while underwater