“This guy no longer looks so tough.”― In-game text
There is only one quest for which collected skulls are needed. Otherwise, if you collect skulls, you can just sell them to the alchemists. You don't know why they need so many skulls. Then again, you don't know what the ingredients are for any of the alchemist potions you buy and drink.
This is a Common item, a frequent drop or obtainable from merchants.