Here is the full list of all 21 Grotto achievements.
Unveil a constellation.
Unveil half the constellations on the sky.
Unveil every constellation there is.
Throw the bones a certain number of times.
Smoke the pipe a certain number of times.
Play the strings a certain number of times.
Choose green over blue.
Choose blue over green.
Prioritize flesh over rock.
Prioritize rock over flesh.
Save the life of a Good-for-Nothing.
Let a traveller in distress share your roof.
Tell the kids a story the right way.
Behold how an icon turns to ashes.
Meet a new tribe and learn a new language.
Give The Brutes their real name.
Seriously, who can kill a child?
Punish the Despicable Liar as many times as ey deserves.
Lose a friend.
Save the life of someone who has lived for quite a long time.
Make a merchant happy.