I'm going to assume that Klaus hasn't told any of you about the Hollow. The Hollow is the spirit of a witch, Inadu, a very ancient and powerful being. She created the werewolf race through the power of her hate and rage. Every part of her seeks more power and destruction.
The point in time that Klaus and I are from, she had taken over Hope. In order to defeat her, we split her spirit into four pieces and placed them in the only vessels strong enough and eternal enough to contain her, and then those vessels were scattered across the world.
You're smart enough to follow that by vessels I mean Mikaelson Originals.
Klaus is from that time, and has the part of the Hollow's spirit within him. When I arrived here he told me that she whispers to him, whispers her hate and plans for revenge, apparently curses my name which is actually awesome to know.
The Hollow gains strength through followers, magical followers, calling to them through her whispers. Rebekah is a Mikaelson witch, in the body of my former wife. Eva was a devout worshipper of the Hollow. As was I, once upon a time. If Inadu tries to reach out and infect people here, the first person she would want to turn is Rebekah.