Sacred Quest

To begin the Sacred Quest you need to visit the Shrine of Singularity.

Once you are there, you need to speak the mantra "unorus".

You will then be asked a few questions, and will fail if you don't answer them correctly.

The answers are from a book you can read at the end of the path straight south from the Tomb of Kings entrance.

Answers:

You will now be able to pass the Guardians at the end of Tomb of Kings, and enter the Stygian Abyss Dungeon.