I need one of these for my enhanced Hybrid Oil, it's the only missing ingredient. But I have no idea where to even start looking for one. Where can I find an Erynia to harvest for ingredients?
Find an Erynia Eye
the-witcher-3
Related Solutions
I found a recipe for Petri's Philtre on the first floor (up the ladder), at Drahim Castle, in Velen.
From the pictured Quick Travel point, walk towards the left of the castle. You will see a doorway that can be broken with Aard or sword. Walk through the doorway, and climb up the ladder found within. The formula is in a lootable box near the next ladder up.
This is likely one of many locations to find this formula, and it's possible that it was a randomly generated loot item.
I'm lv9 and I've never even seen this 'Full Moon' potion. According to the IGN game guide, there isn't a 'Full Moon' potion.
The Wikia states that the Swallow potion recipe is given to you in the Prologue.
Geralt receives this potion from Vesemir in the Prologue during the Defending Kaer Morhen quest.
So you should already have the Swallow recipe available, but you'll need to craft it using the correct ingredients.
Here's what you need:
Drowner brain (1): This ingredient can be found by killing Drowners, which are blue humanoid sea monsters that are usually be found near water. Common locations in White Orchard include the southern shoreline near the ransacked village, the swamp lands to the south of the Nilfgaardian garrison, and along the shoreline to the left of the Ford signpost.
Dwarven Spirit (1): This alcohol can usually be found in Smuggler caches and treasure chests or bought from any White Orchard merchant for 47 coins.
Celandine (5): This herb is super common and can be easily found in the environment. An easy place to find them is outside the herbalist's cottage. They can also be bought from the herbalist for 5 coins each.
This is what Celandine looks like.
It might also be worth updating your game, as v1.04 patch has been released.
Best Answer
They're indeed in Skellige. On the south-western island called Undvik there's 2 caves right next to Clan Tordarroch Forge. If you have the quest called Master Armorer, set it on track and the marker will lead you to the caves' location. Simply enter the other cave which is irrelevant to the quest. At first you will encounter devourers, then harpies and then if you make it all the way down - 2 Erynias. They're lvl 13, but quite easy if you time your parry as they descend to strike.
Been spending 2 days looking for them myself. Glad to be of help!