Runic key is a quest item and key in The Witcher 2: Assassins of Kings.
It can be found on Thorak's body if you are forced to kill him during the quest Baltimore's Nightmare, or in a chest near the south end of the old quarry, though that chest can only be opened using Baltimore's key. This runic key then opens the door hidden behind some vines in a rocky alcove near the wreck of the Eyla Tarn.