Klaus and Marcel are in trouble. After learning the identity of the evil power coming for Hope and the other children in The Originals "Keepers of the House," the two kings of New Orleans might have become the prey of the Hollow, the dark spirit infiltrating the town. In their attempts to save the children from becoming human sacrifices, Marcel and Klaus put themselves at risk and now the Hollow might be in both Marcel and Klaus on The Originals.
Identified by Hope and explained by Vincent, the Hollow is a dark spirit that sometimes takes form in blue light, as seen earlier in the season when it took Adam and Hope. Waking from a dream, Hope tells Klaus that he must never look into a blue light, but even Klaus can't quite help it later in the episode, when he and Marcel stumble into the sacrificial ritual in the woods. Klaus, Marcel, Elijah, Vincent, and Hayley are able to save the kids, but it turns out the entire thing was a trap. The Hollow wanted Klaus and Marcel, trapping them in a ring of fire and transporting them into a kind of other realm, where the only thing visible was a blue light.
Klaus tried to look away, but Marcel couldn't. And now, according to Vincent, they both could have some of the Hollow inside of them. The episode ended with Marcel looking into a mirror, blue light reflecting back on to him, and Hope having a nightmare about Klaus, bright blue rings in his eyes.
The Hallow isn't coming, it's here. And it's in the two most powerful creatures in NOLA.