Priest in Peril
1) Start the quest by talking to King Roald.
King Roald informs you that the defences of Canifis are weakening each day. With the temple priest Drezel kidnapped by Zamorakian monks, the barrier of the River Salve will no longer be strong enough to hold without maintenance.
King Roald will tell you to find these monks and find Drezel, by checking out the abandoned church near Taverly, North of Falador.
2) Knock on the abandoned church door (North-west of Doric's house), to talk to the monks inside. You must impersonate a Zamorakian monk called 'Trevor' who's currently out on a mission.
To do this, you must choose the aggressive options, such as "open up" and "I'll beat you up if you don't open the door".
They'll be convinced and let you in.
3) Prepare for combat
Once entering, two Zamorakian monks will spawn and are HOSTILE. They deal minor damage, so if you're fully equipped, you should be ok.
Kill them to obtain the key and make your way up the stairs in the middle-end of the church. Once upstairs, head to the jail and talk to Drezel. NOTE:If you go up before you have the key, Drezel will tell you to find it on one of the Zamorakian monks below.
Talk to Drezel with the key in your inventory and he will tell you that he needs to head back to the Paterdomus crypt in Silvarea. You will travel with him and be teleported to the temple.
4) Once you arrive in the Paterdomus crypt talk to Drezel, who will ask you to get him 50 rune essence.
5) Return to Drezel with 50 rune essence (if you hadn't gathered before, you're best off going back to Varrock to mine 50. Just make sure you have 50 inventory slots available). Once you've given him the essence, he will reinforce the River Salve.
Congratulations, quest complete!