Finn Mikaelson (Casper Zafer) has bitten the proverbial dust on The Originals, making his surviving siblings startlingly aware that they are not as invincible as they once thought.

Lucien (Andrew Lees) successfully re-engineered Esther's (Alice Evans) spell that turned her children into Originals with the added bonus of a strain of werewolf venom so powerful it can take out the Mikaelson siblings. It gets worse - Lucien made enough of the powerful serum for two: himself and his unstable lady love Aurora (Rebecca Breeds).

As the Mikealsons mourn their brother who only just began to warm up to his family, they must now face the two vampires who hate them the most in peak physical condition. According to executive producer Michael Narducci, it's the toughest threat the true Originals have faced since their father Mikael (Sebastian Roche). He talked to about why Lucien and Aurora are so terrifying, but who might be the key in saving the family.

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How does the death of Finn affect the Mikaelsons going forward?

Michael Narducci: It's two things. One, there was a glimmer of hope that Finn was redeemable and he could reconnect with family. The key to bringing him back in the fold was his love for Freya. It's clear to me that Freya deeply, deeply loved her brother. This was the sibling she had met as a child and always felt a connection to. That was her motive to track down her family for a thousand years, to see Finn again. Then she gets to him and he's back, and he dies. She's devastated. Klaus and Elijah are looking at each other and thinking, "Are we actually sad that Finn is dead?" They are, because at the end of the day it was proven that even Finn could be swayed by this love of family and this vow of always and forever.

The other terrible thing they've realized is that it's not just the White Oak that can kill them now. ... Lucien has combined the magic of Esther, the power of Mystic Falls, Freya's blood, the ancestors power, the White Oak bullet and the venom of these werewolves that has been reverse engineered from the seven packs. He's made himself into something that is un-killable and unstoppable. This is the most jeopardy the Mikaelsons have been in since Mikael was alive and chasing them with that stake. Now they have to decide how they're going to decide to this threat.

Does having reverse engineered this spell open the door for more people to become Originals or is Lucien it?

Narducci: He seems to be the last one with the caveat that he created two doses. One for himself and one for someone else, and I think we can guess by the end of [last week] who that other person will be. He was always in love with Aurora. He wants to share this gift of immortality and power and lethal bite with Aurora. If we're afraid of Lucien because Lucien hates Klaus, then God we have to be afraid of Aurora because she is the unstable character we introduced this season. She is incredibly dangerous — not only to Klaus but to Elijah and everyone else.

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Since he only makes two, does that mean Aurora's brother Tristan isn't really a priority for him either?

Narducci: Lucien does not care much about Tristan. Remember that Tristan is the one that carved a smile into his face and was a complete jerk to him. He might care about locating Tristan almost as a means of wooing Aurora to feeling something for him romantically. He may say, "Look, I've created this serum for you. I have great affection for you. I want you to join me. As a gift, because I love you, I will even go so far as to track down your brother. But as far as who I trust as a vampire to go beside me, that's only you babe." That's the nature of his romantic gesture.

Since werewolf venom is the key to Lucien's new upgrade, can we assume that Hayley will have a key part in undoing the effects of the serum?

Narducci: Somewhere out there, there are seven werewolf bloodlines that are traced back from the original strain of werewolves. The means of producing this venom that is so deadly it can kill an Original goes back to those seven bloodlines. So, for some time now, Kingmaker Incorporated has been hunting wolves, finding wolves and putting together this very, very deadly strain of werewolf venom that is incurable. Lucien combined it with magic and now he's deadly. ... You're right to assume that looking at all the seven packs and beginning with Hayley and trying to figure out what she can do to tap into werewolves everywhere is a great place to start.

The Originals airs Friday at 9/8c on The CW.

(Full disclosure: is owned by CBS, one of the CW's parent companies)