This page offers a guide on where to locate and how to complete The Sky Thief, a mystery found in Cent, AC Valhalla.
Retrieve Mother’s Ring
This mystery is located on the border between Suthsexe and Cent, south of Tonbridge Monastery. Speak to the girl atop the tree and she’ll ask you to help her retrieve her Mother’s ring that was taken by a bird. The ring can be found in a nest in the tree opposite. You can’t climb up onto the other tree, so you’ll need to go beneath the tree’s nest and shoot upwards so that the ring falls out of the nest.
When you pick up the ring, the girl’s step mother will come and speak to you. You’ll choice to either give the ring to the girl or give it to the step-mother. The choice has no real consequence, however it does seem like a nicer option to return the ring to the girl. After picking an option, speak to the girl again to complete the quest.