It turns out Marcel's new girlfriend, Sofya, has her own agenda onThe Originals. A loyal ally to Marcel, Sofya emerged as a new threat to the Mikaelson family in The Originals "I Hear You Knocking" and she just might be working with the Hollow. So, what is Sofya up to on The Originals? The good news is she doesn't seem interested in the Hollow. The bad news: she wants Klaus dead.
As Marcel's most trusted adviser, Sofya got a front row seat to how he reacted with being infected by the Hollow after stopping the child sacrifice last week. So, when Vincent went MIA and an increasingly murderous Marcel was in need of a witch, she provided one. Marcel trusted Sofya's witch, who told him that to get rid of the Hollow he had to bury magic beads in the place where it lives (the abandoned house where it lured children). There, he had a showdown with an also infected Klaus — the Hollow wanted Marcel and Klaus to kill each other so it could feed off their power. In the end, both Marcel and Klaus were strong enough to overcome the Hollow, but it was revealed that Sofya and the witch were both using Marcel. One for the Hollow, the other for revenge.
Sofya wanted Marcel to kill Klaus, but the plan went sideways, ending with the Mikaelsons taking Marcel and keeping him prisoner (because Mikaelsons don't know when to quit, apparently). None of this was part of the plan, Sofya tells her witch, but then they find a silver lining. Because Marcel's super hybrid blood was spilled in his fight with Klaus, the Hollow took his blood as an offering. In return, the Hollow gave them a vine growing from Marcel's blood, and that vine can kill an Original vampire.
Sofya has what she needs to kill Klaus thanks to the Hollow, and Klaus doesn't even know she's coming for him.