Iron Maiden is a Skellige side quest with suggested level 19.

You can pick this quest up from Jutta. She is directly east of Harviken. After talking to Jutta, she wants Geralt to find a sword from a sunken ship, and then she will challenge him to a fight.

Head west from Harviken towards the map marker. Once you are close, you will encounter multiple Ekhindnas. Once you have dealt with them, dive into the water, use your Witcher Senses to find and loot the sword.

Once you have the sword, head back to Jutta and give her the sword. After giving the sword to her, you can then challenge her to a sword fight.

After defeating her, she will invite Geralt to her home after dusk. Head towards her home and meditate until dusk, then talk to her. There is a romance option available here, or you can tell her to try searching the other isles for a man who can beat her in a sword fight.