Start by talking to King Roald in Varrock Palace. Ask him for a quest, and he will tell you that he hasn't had any contact with Drezel, a priest living in Paterdomus, the Temple on the Salve, in a few days, and would like you to go check on him to make sure things are OK.
Once at Paterdomus, right-click Knock-at the door. Someone asks you who you are and what you want, reply that Roald sent you to check of Derezel. Some voices will talk behind the door some. Agree to help the person with the dog.
Head north and into down the trap door and kill the dog. The dog cannot be damaged by magic. When it's dead, go back outside the temple and knock on the door again. Now go back to Roald and claim your reward!
After you tell the King what you did, he will be mad at you and explain the dog was guarding the path to Morytania. He demands you to go back and fix what you've done.
Head back to Paterdomus and open the door rather than knocking. Head up the stairs, then climb up the ladder. Right-click "Talk-To Cell Door" to talk with Drezel. He will tell you that you need to find the key to the cell, and find a way to incapacitate the vampire in the nearby coffin. Go down the ladder, and kill one of the Level 30 Monks of Zamorak to obtain a golden key.
Cleansing the SalveGo back down to where you killed the Temple Guardian, and with the key in your inventory, you can unlock the gate and pass through. Study the monuments in the next room until you find the one with they key.
Use the Golden Key on the Monument to swap it for the Iron key. While down there, also use your bucket on the well in the center of the room. Go back up to Drezel. Use the key on the gate to unlock it. Open the cell door and go in.
You will suggest having him bless the bucket of Salve water and using it on the coffin. Go back in the cell and tell him he can now escape. He'll tell you to meet him down by the well. Go down there, and go through the gate to the north-east and talk to Drezel.
He will tell you things are in bad condition, and your character suggests blessing it, then suggests using essence to adsorb the evil potion. He will need 50 essence to fully purify the Salve.
You must bring him unnoted essence, so this will take 2-3 trips. After you bring him all the essence, he will bless them and drop them into the well. He will give you a special Wolfbone dagger as thanks.