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