If Chunji were to use his eyes to analyze the floor, he'd find that this cathedral is somewhat similar to an overcharged leyline, though weaker. It's not as if the Umbralist is using a leyline connected to the tower, but more so that the cathedral has its own separate leyline that was transported here directly. The presence of this cathedral is also almost definitely the reason for the darkness on the third floor, though there doesn't appear to be any kind of ritual ongoing. Basically, it looks more like this cathedral was summoned from somewhere else as a real physical location, and not that it was created from nothing.
As for the man himself, the robes appear to obscure his features, such that all Chunji can see underneath is a dark silhouette. Even with regular vision, his face is obscured by seemingly the same magical darkness as that of the third floor.