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