Nobody knows, It is said to exist, but no one found it yet

Giant Conguering Hero's Grave - Quick farm and repeat sequence just inside the grace - you have to kill a monk and 2 blacked-out gremlins (leap off the ledge past the monk, then lead the gremlins into light and kill them).

Then the watchdog (also blacked out) is at the bottom of the steps. To kill him, just lead the cat-dog murderbeast into the light circle - it will stagger him so you can get in a few cheap hits - then back off and time his attacks to kill him. I hate these beasts, but this made me get better at dealing with them (even if I still hate them).

You can get it decently fast, but can accellerate the drop rate by raising arcane (which is tied to discovery), and putting on items to increase discovery.

I put on silver scarab , and silver tear mask to increase item drop, and used silver fowl feet - took 6 times killing the cat before it dropped (and three fowl feet cause I forgot and teleported back to grace twice like a dumbass), but I got it fairly fast that way.

I've been doing this for about 2 hours. I must horrible RNG. Even using Silver Fowl foot