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