Here is the full list of all 15 Riddles of the Owls Kingdom achievements.
All levels with 3 stars
All Trophies collected
Hints used in 3 levels in a row
Fully restored owl's house
"Vast Step" world complete
100 levels with 3 stars
"Immense Taiga" world complete
Collect all owls in their house
60 levels with 3 stars
"Unassailable Mountains" world complete
40 levels with 3 stars
20 levels with 3 stars
"Endless Sands" world complete
Solved level in less, then 120 seconds
Tutorial finished