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 just seen a lot.
You've built a beautiful cairn.
Finally, it's over.
You've been... thunderstruck!
You've passed the second nightmare. You're almost happy now.
Delicious!
Knives are for cutting, not for licking.
You just make the hearts pop.
Congratulations, you've endured the first nightmare. On to the next one!
How many heads does it have?!
You've found the book of the dead.