A Towerful of Mice - Velen
Location: Northeast of Midcopse
Quest Giver: Keira Metz
Other Requirements: Complete An Invitation from Keira Metz Quest
You can accept this quest after completing the story quest line that involves Keira Metz and the optional quest she gives, Magic Lamp. Completion of this quest is also required to romance Keira Metz or to receive her help later in the game in Kaer Morhen. For this quest you're tasked with visiting Fyke Isle which is located in the center of Lake Wyndamer.
On the island you'll want to use the Magic Lamp near the green fart clouds you come across, you can find them by looking for the Magic Lamp icons on the map and compass. They're the white icons shown all over the island on my screen shot above and on your ingame map. Use the Magic Lamp at each of these locations to learn more about what happened here and to advance the quest.
Don't forget to use your Witcher Senses too once inside the tower - there is a lot to inspect using them as well. On the second to last floor hit the switch to reveal a hidden passage that leads you to the top of the tower where the Laboratory is.
At the top of the tower using the Magic Lamp will allow you to converse with Annabelle (pictured above). After some dialogue you will be given the option to aid her or not; both of which minorly change the rest of the quest but have no bearing on any future quests (but a HUGE bearing on future Witcher lore - unrelated to the game).
I strongly recommend you offer to aid this ghost and bring her bones to Graham as that will drastically decrease how long this quest takes you to complete. If you choose not to aid her you'll have to go see Graham then bring him all the way back to the tower which will take forever. It's easier to just take the bones to Oreton and give them to Graham, it ends the quest right then and there.
Here is the fun part of this quest though, bringing Annabelle back to Oreton actually reveals that she is a pesta and it will unbind her curse from the island. She's now free to roam across the entire world and she'll go to the kingdom of Kerack and spread a disease there that brings down basically the entire kingdom. As I said, this isn't part of the game and is just part of the Witcher lore as a whole, I found it interesting enough to share with you all how much of a butterfly effect Geralt's actions had.
Once you're finished with this quest return to Keira and turn it in. This makes a followup quest available called A Favor for a Friend.