Here is the full list of all 20 Knytt Underground achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Find the hidden ending
Ring every bell in Chapter 3 without paying a guard
Find a hidden recipe
Complete the Mini Figure
Collect a hidden symbol
LAZA KNITEZ!!
Strange things happens around him
Find the hidden sheep
Find music hidden in the sky
The proper way to say "I'm sorry!"
Complete the Figure Space
It has music by D Fast
It's just sitting there for some reason
Find the hidden Juni story in the credits
Complete a hidden challenge in the sky