I might be wrong, but I think the "invisible" boss in question is the Daedroth which must be cleared before Dark Root appears, although in the video it looks like the Infested mobs that accompany the Daedroth are also invisible. The Daedroth has an attack shaped like a cone or wedge which "infests" you, so you must either break free of the wedge AOE or try to avoid it entirely. In the video, you can see the red wedge attack appear every so often.

Dark Root is actually pretty easy to kill. She spawns two adds which drain Magicka (blue) and Stamina (green), and after you kill those adds they leave blue and green areas which you can stand in to gain rapid Magicka or Stamina regeneration, based on whichever resource your character relies on the most, thereby making it easier to attack Dark Root for a while until the blue and green areas disappear and the two adds are respawned.

I've had some rare occurances of bosses becoming invisible in trials for the group, the ones that come to mind are AA first boss and DSR twins, both within the last few weeks, and impacting all players (so not individual ping or local rendering issue)