The Last Wish is a Yennefer side quest in Skellige, with suggested level 15. It becomes available after the main story quest Nameless.

Once you have the quest, meet Yennefer in Larvik to start the quest. The quest involves helping Yennefer find a Djinn.

Head towards the Larvik harbor with Yennefer. When you arrive, jump into the boat with Yennefer. When Yennefer stops the boat, she will cast a spell to give Geralt the ability to breath underwater. Jump out of the boat and dive down, use Witcher Senses to find clues if the Djinn is there. If you encounter Drowners, use your crossbow to deal with them.

After investigating the nearby shipwreck, Yennefer determines its not the right ship. Head back up to ship, and Yennefer will steer the ship towards the next destination.

At the next destination, dive down and investigate again with witcher senses. You don't need to dive that far to investigate the nearby wreck. Geralt determines its not the right ship, so head back up the boat.

At the next destination, its quite a deep dive, Yennefer will look through Geralt's eyes and determine its the right ship. Kill the Sirens with your crossbow. Investigate the ship wreck with Witcher Senses to find the Djinn. Instead of finding the Djinn, you'll eventually find a seal for the Djinn. Head back to the boat and show her the seal.

Yennefer will find the other half of the seal with her magic, and teleport both her and Geralt there.

The other half of the ship landed on top of a mountain. Use Witcher Senses and investigate the ship to find the other half of the seal. The seal will be located on the lower level of the ship, which is accessed down the stairs from Yen. Once at this lower level investigate the shoes, then the blood trail, and then the bookcase. The mage's body will be found crushed under the bookcase. Examine his body to find the other half of the seal.

Head back up the stairs and talk with Yennefer, as she summons the Djinn. You will need to defeat the Djinn. After you defeat the Djinn, there will be a cutscene and conversation with Yen. This completes the quest.