As the most mysterious warriors among the Shadow Cloud Spirits, they possessed the ability to carry invisibility. Of course, such abilities weren't accessible to just anyone, only the top echelon of the Dark Sanctuary Knights had them, and the ones Adriana Wilson had sent to Finn Lewis were naturally among the cream of the crop.
However, this scene terrified everyone in the grand hall, because it clearly involved an invisible enemy, yet, despite having detection devices in the hall, nothing was found; their personal smart devices should have had this capability as well.
Optical invisibility wasn't an advanced ability, but the beings that had just appeared, albeit briefly, were clearly seen by all, and they were not human, but an alien species. This was not surprising to them, as many alien species served under humans here.
