Key to the oldest cell in the Irithyll dungeon.
The first prisoner of the dungeon was a lone giant. Cells for men were built at his feet.
Found at the end of the sewer containing the Mimic, Hound-rats and the Basilisk ambush in Irithyll Dungeon. From the Irithyll Dungeon bonfire, proceed to where you encounter the giant for the first time. Look for the ladder that leads down to the narrow bridge below. Past the bridge is the sewer.