Here is the full list of all 18 Munin achievements.
Complete Múspellsheimr
Complete Vanaheimr
Light up 4 Runes at the same time
Complete Helgardhr
Free 9 Orb spirits
Stand on top a rolling boulder for more than 3 seconds
Complete Nifleheimr
Collect 9 Feathers while underwater
Finish 9 levels without jumping
Complete Midgardr
Complete Jötunheimr
Complete Ālfheimr
Complete Svártalfheimr
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