A quest from a King-

To start Priest in Peril speak to King Roald, who can be found in Varrock Castle. He will say that he has lost contact with Drezel, a priest who lives in a temple to the East. Agree to check up on Drezel to start the quest. Then head to the temple as seen on the map below to continue the quest.

Suspicious Activity-

Right click on the temple door and choose the knock option. Tell them Roald sent you to check up on Drezel and a suspicious conversation will begin. Someone claiming to be Drezel will ask you to kill a horrible dog under the mausoleum to the North. Climb down the ladder and kill the Temple Guardian, then head back and knock on the door again. The suspicious voice will laugh a lot and tell you to report back the king.

It's a trick-

While in Varrock get a bucket and 25 (not noted) Rune Essence to bring with you, make sure you have at least 1 empty slot in your inventory as well. Speak to the King and he will be furious that you killed the dog. He will explain that somebody must of tricked you and that you have to go back and fix your mistake before it’s too late. Head back to the temple and go inside; the door will be unlocked now.

The Golden Key-

Kill the level 30 monks until you get a golden key, then head up to the third floor of the temple. Right click on the cell door and select the talk through option to speak to Drezel. He will tell you that you need to stop the salve from being ruined. He will then explain that the key to free him must be nearby, but if he passes by the vampire coffin near the cell it will wake up and kill him, due to his ancestors.

Halfway there-
Now head back to the mausoleum and go through the gate past the dog. Study the monuments inside until you find one that shows this.

And then use the golden key on it to switch keys. Head back to Drezel and use the key to unlock the cell. Speak to Drezel again and he will tell you that the water of the salve might work against the vampire. Head down into the mausoleum and use your bucket on the well in the middle of all the monuments, then go back to Drezel.

Problem solved-

Speak to Drezel to get the water blessed then use the bucket on the coffin. Then speak to him again, he will tell you to meet him down in the mausoleum, in the room past the monuments. He will tell you that he might not be able to repair the damage. You will tell him about Rune Essence and he will ask you for 50 of them. Speak to him again to hand him the first 25, then go back to Varrock and get the rest. Once you have handed him all 50 he will give you a dagger that stops werewolves from transforming, and the quest will be complete!