Original written by Aribeth the Ranger:-

Speak to Caldoron to investigate an earthquake in North Kytra Province.
The crevasse that you must get to is just slightly north from Lion's Arch. It is to the east of Miraba and Leighton Cranford. Click on the crevasse and choose to enter. You will brought to Beneath Lion's Arch. Follow the path through the small cavern and into the ancient hall, killing some undead along the way. Keep going on the singular path until you see dwarven allies. Climb up the stairs and a cutscene will begin. This quest ends and is completed automatically when the cutscene ends. The Beginning of the End is the follow-up primary quest that is automatically added to your quest log