As far as attacks went, this was one of the more perplexing. Stealth was best served by thievery or decisive strikes. While it may have given a momentary advantage, that advantage diminished once your presence was known. Still, Damian didn't hear anything that didn't belong, no sounds of a stranger breathing or foreign footfalls.
He hadn't moved quickly enough to intervene so there was no reason not to press the attack on Loki. He was about to express his confusion when Loki disappeared.
There was a quick glimpse of a widening hole as if the carpet and floorboards didn't even exist and then Loki was falling. Had he not caught a glimpse of the library, Damian would have assumed it was some sort of teleport spell. At least this part made sense. Divide and conquer. It was an old strategy.
Ophelia looked about, then circled the area where Loki had vanished. She was still at ease but no longer openly confrontational. Titus, on the other hand, just seemed confused. Damian took off towards his door and after a moment Titus followed. It was obvious that Loki wasn't going to be reappearing soon.