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