Kingmaker

 is an achievement in and requires one to complete the secondary quests that'll determine Skellige's new ruler.

Tips

 * After the main quest The King is Dead – Long Live the King, you'll automatically be given two side quests: Possession and The Lord of Undvik. You must complete one or both of them for the next part.
 * Do King's Gambit and side with either Cerys or Hjalmar (if you only did one of the other side quests, only that person will be available).
 * You must side with one of the siblings, otherwise choosing not to help will lock out the achievement.
 * Afterwards, do Coronation and, once complete, this achievement will unlock.