The Kidnapped Princess
During the quest "The Hobbybrothers", you will have found this place in search of a book. This place has its own quest though.
dad is worried
Here you will find a man named Walter. He is worried that his daughter is missing. He thinks she may have been kidnapped. Tell him that you will help him find his daughter to begin the quest.
You will have to search for clues on his daughters whereabouts. Inside is book about a "Princess". In the book, it mentions the princess running away to a cave.
Go to the quest marker shown on the map to find the cave.
Past a few enemies you find a trapdoor hidden in the woods. Go inside.
Here you will see Bella having fun with something she calls "Glowy".
Before speaking to her, you may loot the cave of various items. The loot of importance here is a rare staff called the Staff of the ember Magus. It is a good weapon for a mage although its requirements may be a bit far from what you will probably be in when you run into this quest.
Speak to Bella when you are done looting. Tell her that her father is worried and you will have an option; You can convince her that you will take care of Glowy, or tell her that her dad will let her keep Glowy.
At the time of writing, there seem to be no difference with the choice you pick and it doesn't seem to have any impact further down the line.
After convincing her to go back, speak to Walter to get your quest reward and complete the quest.