An Innocent Kid
It was true that the Ichor Blindness had the power to impair his vision, transforming all non-vampire entities in the vicinity into a crimson distortion. It was also true that the Vampires' presence was erased, undetected by the ordinary senses.
However, Rex has a seventh sense that was an accidental benefit of the System.
Caught off guard earlier, he was battered by the Vampires.
Unfortunately for the opposers, Rex's ingenious solution to counter the Troubadour of Blood War's influence didn't take long to manifest. All he needed to do was to scan their stats, and he did exactly that, telling the System to scan all of the entities within his vision.
Due to that, several stats materialized, orbiting the invisible Vampires.
He now could tell where their location would be without the need to see them directly.
Rex was able to block the incoming attack because of this.