Gargoyle Skulls are the skulls found on Gargoyles from before the Searing.
Drop and salvage research is ongoing, and can be found here.
Enter the Catacombs from Ashford Abbey. There's a hidden path behind where Munne stands, follow this path to find an area heavily populated with Gargoyles. Follow the path to the west (you'll encounter more gargoyles along the way). At one point the road splits to the west and to the east. East is a great hall with 5 gargoyles down the stairs and 3 more a bit further away. West is a big bridge over some kind of dragon skeleton and leading to the exit to Green Hills County with around 9 gargoyles. Shatter Gargoyles (4 in total, in remote locations) possess Backfire, which can deal around 50 damage to your character each time you cast a spell. Refrain from using spells when hexed if you're low on health. Alternatively, use a martial weapon (not a wand or staff) since the AI only cast it on characters using a caster weapon.
Location: The Catacombs
Collecting: 5 Gargoyle Skulls
Location: Regent Valley (pre-Searing) in the secret garden.
Collecting: 5 Gargoyle Skulls (on certain days)