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