Here is the full list of all 15 In the Service of Mrs. Claus achievements.
Find a job in the mortal world.
Win a war decisively.
Witness the birth of a smaller piece of you.
Learn the truth about Santa's death.
Betray Mrs. Claus and the rest of the elves.
Become the head of a cult.
Have your heart's wish granted by an extradimensional being.
Destroy Christmas.
Be the last person standing at the end.
Meet Shank Zappa.
Deliver presents in Santa's sleigh.
Befriend Cerberus.
Celebrate your wedding day!
Bring someone back from the dead.
Save Christmas for future generations.