Here is the full list of all 18 Solstice achievements.
Experience all possible endings.
Abandon your goals.
Leave a mark on the wasteland.
Seek magic and power hard enough.
Let Yani be the doctor for once.
Just relax and enjoy the baths.
Save the day like a true hero.
Receive assistance from someone who trusts no one.
Get intimate with an ex-killer.
Make a promise to someone you barely know.
Let the Winter win.
Keep it warm and natural.
Make the final choice.
Bring the murderer to justice.
No dog should see this...
Find out what happened to Lev.
Play it safe when talking to strangers.
Pay attention to Galen's mentor.