I Hear You Knocking

Klaus and Marcel, under the influence of the Hollow, are driven to a dangerous game, tasked with killing each other. Freya successfully develops a knife to kill Marcel while growing closer to Keelin. Hayley visits Mary to keep Hope safe, where she learns that Mary's husband had journals including the serpent sigil that Hope has drawn and is all over town. Sofya and Dominic share that The Hollow was let free when Vincent, Davina, and Kol severed the ancestors' link. Hayley later begins to question if the cult of The Hollow made Mary's husband kill her parents years back. Elijah and Freya barely save Marcel and Klaus from killing each other, just as the blue orb begins to form a black shadow of a person. Elijah keeps Marcel bound in the dungeon, fearing Marcel will halt Klaus' redemption. Sofya and Dominic meet at the battleground, with Sofya angry that their deal was broken; Klaus was not killed and Marcel was taken. Dominic points out that Marcel's blood had been spilled and they can use it to kill Klaus or any Original.

Plot summary from Wikipedia (CC BY-SA 4.0). Source