If you use the ballista on the right side of the main platform of the Skittering Breach to reach a necropolis floating far up above the left side above the platform, and the drill sergeant is still alive doing a patrol on the main platform (nowhere in aggro radius), the sergeant eventually teleports on top of you while you are inside the necropolis fighting the mobs there. Maybe when he reaches the ballista you used to shoot upstairs on his patrol? I could not see the sergeant’s location when it occurred. This was a tier 11 of “Relics of the Old Gods” storyline.