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