The Kos Parasite is the final weapon you can obtain in Bloodborne: The Old Hunters. It’s also the creepiest. In order to obtain it, you have to beat the game’s final boss and you’ll get it automatically.
There’s also a trick to using the Kos Parasite. If you equip it normally, you won’t be able to see it on your hunter. You have to transform into a Kin before you can use it. To do this, you have to obtain the Milkweed Rune from Adeline in the Research Hall. Equip the Rune, then equip the Kos Parasite, and you’ll become a tentacle-wielding monster.
“When the carcass of Kos washed up on the coast, its insides were teeming with tiny parasites, unlike any found in humans.
This atypical weapon can only be clasped tight and swung, but a Kos Parasite is said to stimulate phantasms inhabiting a lumenwood.”