A Sinister Plan
Speak to the monk standing outside of the Temple of Paterdomus, and he will ask you to construct a very interesting weapon. He tells you to grind down a Mithril two-handed sword on Doric's whetstone, and then to string it with a bowstring.
Do exactly as he asks, and you will be left with an odd-looking bow-sword.
Return to him, and he will give you an orb to put on the altar at Entrana, for a ceremony which he regrettably can't attend. However, he tells you that it's a surprise, and must be snuck in inside a large pouch.
Large pouches can be obtained from Wizard Korvak at the Runecrafting Guild for 25,000gp, or are dropped by various Abyssal creatures (excluding Abyssal demons). Use the Orb on the large pouch, then use the Mage of Zamorak to access the Abyss. Exit through the Law rift (on the northeast part of the Abyss' inner circle) and exit through the portal at the Law altar.
Place the Orb on the altar, and you will view a cutscene in which an Assassin appears and steals a holy relic from the monks.
Making up for it
After the chaos unfolds, the High Priest tells you that the orb you delivered contains very volatile magic, and that the monks' holy relic must not fall into the wrong hands. Return to the Temple and you will find a monk laying on the ground dead.
Return to the High Priest to tell him about the dead body, and he will tell you to report to Sir Tiffy in Falador. Speak to Sir Tiffy in Falador to tell him what you did, and he will tell you that the Temple Knights will take it from here.
