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