Tarnished key is a key and quest item in the Hearts of Stone expansion.
On a body near Arnskrone Castle Ruins signpost, this can be found along with Count Romilly's journal (part 2). Reading the letter starts the quest The Secret Life of Count Romilly. The key can be used to unlock the chest in the first room in the ruin's basement. The other chest is unlocked by doing the quest A Dark Legacy.