I think someway during the last years updates they dropped the level requirements for the main missions. I was around level 20 when I did this mission, but I guess I could have done it earlier. As soon as you complete the previous mission, leave the harborage (the cave where the blind prophet is living) and walk away. After <1 min the next main mission will pop up and you can return to the harborage.
This mission itself is pretty standard. You teleport to a dungeon. Fight your way and beat the boss - in this case Mannimarco - and teleport back to the prophets cave.
The individual steps during this quest are as below:
*** Spoiler - click to reveal ***
Enter the Harborage.
Talk to Varen Aquilarios.
Talk to Sai Sahan.
Wait for Varen to open to portal.
Enter Portal to Sancre Tor.
Explore Sancre Tor.
Confront Mannimarco.
Talk to Sai Sahan.
Go to Sancre Tor Inner Chamber.
Confront Mannimarco.
Defeat Mannimarco Minons.
Continue to the Inner Courtyard.
Wait for Sai to Catch his Breath.
Talk to Sai Sahan (Optional - Talk to Lyris)
Enter the Dragonguard Tomb.
Consecrate the Shrine in the Dragonguard Tomb.
Go to the Reman Vault.
Consecrate the Shrine in the Reman Vault.
Continue to the Vault Antechamber.
Confront Mannimarco.
Defeat the undead.
Enter the Vault of Kings.
Talk to Sai Sahan.
Open the Vault Door.
Defeat Mannimarco.
Collect the Amulet of Kings.
Return to the Harborage.
Talk to Varen Aquilarios.