Here is the full list of all 18 Solstice achievements.
Let Yani be the doctor for once.
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.
Seek magic and power hard enough.
Bring the murderer to justice.
Let the Winter win.
Keep it warm and natural.
Make the final choice.
No dog should see this...
Get intimate with an ex-killer.
Find out what happened to Lev.
Pay attention to Galen's mentor.
Just relax and enjoy the baths.
Abandon your goals.
Experience all possible endings.