[s4e5] I Hear You Knocking -

Watch the official inside look for this episode to see the producers discuss the Hollow's psychological grip on Klaus and Marcel:

: Though the intended sacrifice failed, the Hollow accepts Marcel’s blood and grants his allies a magical thorn capable of killing an Original. [S4E5] I Hear You Knocking

: The Hollow manifests as his abusive father, Mikael. These hallucinations taunt Klaus, suggesting he is weak and that the only way to protect his daughter, Hope, is to revert to his most brutal, murderous self by killing Marcel. Watch the official inside look for this episode