One of the coolest weapons in Bloodborne has got to be Tonitrus. Here’s a video showing the location of this bitchin mace. Keep in mind that the Hypogean Gaol lamp becomes available after The Blood-Starved Beast by letting a Bagman kill you, you will find yourself in a cell. Work your way to the set of double stairs and go up the left side, the lamp is here. Remember that if you want Tonitrus then you must pick it up before killing Rom, otherwise your going to have to waste some blood echoes and purchase one from the messenger shop, who will have it for sale after you acquire The Spark Hunter badge from Darkbeast Paarl.
Bloodborne: Tonitrus Location