Vacca is a non-rabid Dragon Elf who is located on Sentinel Island and gives the quest Ghostbuster during the events of Divinity II: Ego Draconis. Lately some ghosts have taken over his cave and he is too scared to fight them. He asks you to go there and clear the cave and as a reward he will give you a potion which fools the wyverns sense of smell, making them friendly for the quest From Soup To Nuts.

While the other Dragon Elves were driven mad by the Laiken's cruel reign he retreated to his cave in solitude and knowledge of the books.

You can mindread him for 700 exp to learn where the hermit hides his secret bow.

In DKS when killing him, he has a heroic and sometimes even Legendary (like 25%) jewelry drop slot so you can use this opportunity to kill him & save/reload for an early neat jewelry piece.

Loot his gem in his cave it's worth 5000 gold at a trader (this gem will be gone after you complete the Ghostbuster quest).