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