Meet Poturi on the Plains of Jarin, just outside the Sunspear Great Hall. With him, follow the quest marker in direction of Champion's Dawn to a place on the southern coast of Jarin. Here you will find a large group of 10+ corsairs. One of them is labeled "Wounded Corsair" (he is squatting). Kill him, then open a nearby Corsair Chest (no key is required) to retrieve Poturi's Notched Sword. Hand it over to him and you're done.