Elven-made key is a key and quest item in The Witcher 3: Wild Hunt, which can be found in the Ancient Crypt on Ard Skellig, on a body impaled by spike traps, at the hidden treasure marker next to the room with the venomous arachas, after the crypt has been opened during The Sunstone quest. It opens the locked chest on the other side of the traps.
It is technically tied to the quest, but Geralt can come back to loot after it has already been completed.