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