I have beat this game a dozen times and have never encountered this item.
It's crazy how much time you can spend on this game and somehow still managed to discover something new on a playthrough.I found this in level Three of the Great Pthumeru Ihyll Chalice dungeon, just laying around on the floor. The item is too pretty so I don't think I'll ever use it.
They're semi common in depth 5. Although my favorite thing about bloodborne is its ability to make me go "what the fuck?!" after countless playthroughs