You must locate, defeat, and capture three spirits for the Necromancer. You will meet the Necromancer while following the main story. Note: The ghosts can be gathered in any order.
One is located in the Rise at the entrance to the Lava Grotto. Use Creo+Move with the Gaea element to cross the lava to the right. You will come across a chest with green soul beads and the tomb can be found just a little farther past it.
The second ghost is found in the Highlands. To the left of the entrance to the Tomb of the Mage-Kings is a large statue; behind it is a floor switch. Use Ego+Duplicate on the switch to grab the key inside the statue. Head to the left and after crossing a wooden bridge the tomb will be down a small hill next to the river.
The third and final ghost is found in Greyleaf Hamlet. After the village is burned down head to the upper left of the area where the entrance to Mystral Woods is located. A metal gate that was previously blocking the entrance to the graveyard in the left of the area is now destroyed and the tomb is found near the entrance to this graveyard.
Head back to Mystralia Castle in the Highlands and head to the docks to find the Necromancer to finish the quest.