The Beasthunter Saif is one of the many new trick weapons you can acquire in Bloodborne: The Old Hunters. To get it, teleport yourself to the Nightmare Church lamp. From here, instead of heading right to the path that leads to Ludwig, turn left and go past the 2 hulking Cthulhu-like monsters. Climb up the ladder at the end of the blood river.
After you’ve reached the top of the ladder, turn right to see a sectioned-off area you can jump to. There are gravestones and blood everywhere. You should also see a glowing item near the ledge of the section. Jump down, but be careful of a blood-addled hunter hiding in the corner. He hits hard, but if you can dispose of him, the Beasthunter Saif is yours for the taking.
“A trick weapon used by the old hunters.
A second blade is found inside the curve of the main one. In its initial form the saif can be wielded like a long curved sword, but when transformed, its blade is contracted allowing for quick, repeated stabs.
Allthough this trick weapon allows for adaptive combat, it was later replaced by saws and similar weapons that were more effective at disposing of beasts.”