Elijah and Vincent formed a tentative alliance on The Originals in the hopes of finding the Hollow, but they weren't as successful as they might have liked. In "Bag of Cobras," Vincent decided that the threat from the Hollow was greater than his hatred for the Mikaelsons and agreed to help track down the evil spirit threatening NOLA and Hope. Thus, Elijah and Vincent hunted for The Hollow on The Originals and decided the best way to do that would be to throw a party.
Elijah and Vincent lured all the major supernatural players to a lavish party at the Mikaelson compound so that Vincent could get a read on them and figure out who was working for the Hollow. There, they found Dominic, the witch who was working with Sofya. At first, they just wanted to ask Dominic questions. Elijah had made Klaus promise there would be no bloodshed at the event, so it was interrogation only — at first. Dominic tried to make a deal with Elijah: give the Hollow Marcel and Vincent (aka sacrifice them), and the Hollow would spare the Mikaelson family. Dominic made clear that the Hollow was not backing down, and, seeing he wouldn't be able to reason with him, Elijah killed him in front of the entire party. Turns out he's more lethal than Klaus these days.
Meanwhile, Klaus learned about Sofya's grudge against him — he murdered her family 500 years ago — and traded her thorns that can kill an Original for Marcel, only he failed to deliver. Both he and Vincent weren't particularly thrilled with Dominic's public execution, and Vincent made one thing very clear: his alliance with the Mikaelsons is temporary. After they defeat the Hollow, he wants the entire Original family gone from New Orleans — for good.
Other than bringing Elijah one step closer to becoming his brother, killing Dominic didn't really do much. At the end of the episode, the witch was revived by other followers of the Hollow, and he's not going to stop until he gets what he wants.