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