There are two methods for completing this mission. You can either race through it and try to drop three Light of Seborhins on the Harbinger of Nightfall before the friendly NPC dies or you can circumvent the trigger on the floor which causes the friendly NPC to move locations and die. This article will discuss the method where the trigger is circumvented.
Mission
As soon as the mission starts, pin your hero, and grab the Light of Seborhin. The tendrils to the left can be pushed through, as shown in these images:
Once on the other side, place the Light of Seborhin on the bridge, as shown in the image below:
For the next 15 or so minutes, spawns of 4x Margonites will appear every 2 minutes until there are 28 of them. As they roam around, you'll be able to aggro a caster and Death's Charge back down from a specific location, as seen in the images below:
Once downstairs, you can run past all the mobs. Pick up the Light of Seborhin from below the bridge as you run past. Once the groups of 4x Margonites stop spawning, you can use your Light of Seborhin to kill a Harbinger on the North or South to spawn more of them. Drop three Light of Seborhins on the Harbinger of Nightfall to end the mission.
Bonus
Video links
Shiburito R/A
Nao Me/A (with bonus)
Serial E/A
Shade A/E
nightquist pure D