A book, called Report Of The Toyseller, can be found in one of the desks in magisters' hideout. Read it and you will learn about a man called Toyseller.
Visit the man and speak with him. You learn that he can help you pass a ritual in The Cathedral. However, he can refuse to help you if your Persuasion is too low.
This can change if you free Lord Arhu. Inform Toyseller about that and he will decide to help you. The man gives you Source Amulet and also tells you to take a scroll from a drawer located on a floor in his house. You get a password that will allow you to open it.
Reward: 150,850 experience points