The Witcher 2: Assassins of Kings
This item is dropped by the queen harpy and other kinds of harpies.
Associated quests
Note
In the journal entry related to "An Encrypted Manuscript" quest, it is said that this ingredient can be substituted with a Rotfiend tongue as well.
The Witcher 3: Wild Hunt
Harpy eggs is an alchemy ingredient. It is not needed to craft anything but can be dismantled into monster egg and monster feather.
It is a random drop from harpies and guaranteed from their monster nests.
External links
See the GWENT standalone game version card: Harpy Egg