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