He will ask you to find out what happened in the lighthouse that has recently gone out. To unlock this quest, you first need to complete the one with the spiders (it is recommended that you first complete the ‘Bandits’ quest for the next level.) You it by going to the sheriff in the garrison building (1). With the campaign completed you’ll receive a Broken Awoken Talisman, a small item that you need to reach the Dreaming City. How to Start the Key of Light and Darkness Quest To kick off the quest you’re going to need to talk to Petra Venj after completing the main campaign for Destiny 2: Forsaken.