Here is a map of where you need to go to start the quest to get your creature. The cave is by the two shrines on the map at the top western point. The necromancer in the cave will give you a quest called Vigor Mortis. To complete the quest go to cave 2.12 on the map above, kill the three creatures in the cave, and collect the crystal skull. The cave is near the fortune teller (southernmost point). Bring the Crystal skull back to the necromancer and he will let you make your own creature and then all you have to do is summon it by clicking on the crystal skull he gives you which is in your inventory.