Search the house next to the Gatekeeper and find the cause of the tremors.
Defeat Lucius, the cause of the tremors, and bring proof to the Gatekeeper.
To get to the Gatekeeper, you most first go to Lowtown. Once you arrive, you must then travel East until you get to a corner. Once you hit that corner you should then turn North until you get to another corner, where you will turn East once again. If you look around you will see the Gatekeeper standing in front of a gate where he will be waiting to give you The Tremors.