Here is the full list of all 11 Happy Game achievements. It takes around 1-2 hours to unlock all of the achievements on Windows.
You've built a beautiful cairn.
Delicious!
Knives are for cutting, not for licking.
You just make the hearts pop.
How many heads does it have?!
You've just seen a lot.
Finally, it's over.
You've been... thunderstruck!
You've passed the second nightmare. You're almost happy now.
Congratulations, you've endured the first nightmare. On to the next one!
You've found the book of the dead.