No topo, perto dos NPC do Patrigio e Kansia

Directly in the center of the island. You can see the quest NPCs on the right of your minimap. Look for shady mist in a sunken area at very top of mountain.

https://i.imgur.com/nq5pUAI.jpg

If you found Ancient Gaudian Slate, there is a crater on your east, just go down into the center of it. As mentioned on other comments it is on the center of the island, out in the open (not inside).