The Bloodletter is one of the most badass looking weapons you can acquire in Bloodborne: The Old Hunters. In order to get it, you have to first obtain the Underground Cell Inner Chamber Key to open the lowest cell there. You can get the key from Simon at the Lighthouse Hut lamp, along with the Bowblade.
Head back to the underground cells where you’ll have to fight an invader version of Brador, the Beast-skinned Hunter. He will occasionally consume a Lead Elixir, making him impossible to stagger, so be sure to keep away from him until it wears off. You can also attack him right before he consumes it. After taking out Brador, head down the stairs to the cell at the very end. Open the door with the key and you’ll find the real Brador waiting for you. He won’t put up a fight, so just strike him down with your weapon and examine his corpse to get the Bloodletter.
“The demented hunter weapon brandished by Brador, the Healing Church assassin.
The Bloodletter assumes its true and terrifying form after it draws upon blood from the inner reaches of one’s body and soul.
This is the only effective means of expelling tainted blood, or so Brador, isolated in his cell, continued to believe.”