The Old Hunters is the first DLC or Expansion for Bloodborne and with the release of the first DLC you can now join the covenant and recruit NPCs which will help yu with earning new rewards along with defending you with Multiplayer battles. With this Bloodborne: The Old Hunter Guide, you will earn the Impurity Rune and unlock the Rune Workshop Tool.
Alos, Check out our Guide on How to Go Beast Mode.
In the Old Hunters DLC, you must be able to join the League which allows to hire the NPCs and play with them in the multiplayer. You will be granted to Impurity Caryll Rune once you join the League, but you need to earn the trust and increase your reputation to gain the Rune.
Find Valtr in order to join his League, he can be found in the Forbidden Woods. As you see the First Lantern move straight and you'll see a hut on the right which leads to the secret road(shortcut). Once you Found Valtr, talk to him and inform him that you wold like to join the League and as you Join you will be awarded the Rune.
Once you Gain the Impurity Rune then the next step is to equip it and for this you need to unlock the Rune Workshop Tool which is located in the same room where you first fought 'Witch of Hewick' in Hemwick Charnel Lane. Once you equip the Impurity Rune, your Max Health will be increased by 2% when you play as a co-op. You also receive Vermin which helps you gain more Status in the current League. Now get going in the World of Bloodborne and Show them what the Power of Rune can do along with your fellow League members.