When Rubella joins the party she will give Aurora her brother Tristis' letter:
"Dearest sister, Rubella, night has fallen on our land. In these dark times one must take a stand. Not to hide up in the clouds, curled. But to juggle the weight of the world. At death's gate to slip and fall, And triumphantly dance around it all. In short: To laugh or not to laugh? That is the question. The answer is clear, I will follow my dream's intention. And join the travelling circus on the plains, before its ascension. Sister, I hope you will not remain listless. Your brother, Tristis"
And give the quest Rubella's Plight. The goal is to find Rubella's brother. To resolve the quest, Aurora has to speak to Tristis in the Cliffs of Erin (first appearing in Chapter 7).
Tristis can be found on an Island to the very left, right beyond the entrance to the temple in the sky.