Warning: Spoiler alert! Do not proceed if you have not watched Friday's season six finale of The Blacklist.

Things are about to get very complicated on The Blacklist. 

After taking down President Diaz and exposing him as a fraud, Raymond Reddington (James Spader) turned his attention to a bigger question from his past, meeting with a mysterious stranger twice to dig up intel on a person of interest. But who exactly was Red after? Turns out, his plan was to reunite with Liz's once-presumed-dead mother, Katarina Rostova, who has only previously been seen in flashbacks.

And in the closing moments of Friday's season six finale, Red made the trek to Paris, France, to warn Katarina that it's not safe for her to be there -- only to get a surprise of his own. After sharing a kiss, Katarina goes in for the kill, drugging her former lover and dragging him into an unmarked black van. Why?

ET spoke with The Blacklist executive producers Jon Bokenkamp and John Eisendrath to break down the biggest revelations from Friday's season wrap-up, including the significance of the finale's shocking last scene, which brought Liz's once-presumed-dead mother Katarina Rostova from the past to the present day.

ET: We find out the real truth about why President Diaz concocted the layered assassination plot-- that it wasn't to kill himself but to kill his wife, in order to cover up a wrong he committed years ago. How did you guys come up with this particular answer to that big question?

John Eisendrath: Jon and I, and I think the other writers on the show, prefer to think that it's almost always a more satisfying story if a big question has a very small answer. If a question about an attack against America isn't answered by "Oh my god, it's the Chinese," "It's the Russians," but instead something very personal and very relatable, something that people could be like, "Oh my god, I totally understand on a simple human emotional level why someone would be willing to do something so seemingly insane." In some ways, it's invariably the more satisfying conclusion to a story. No one is going to suspect that he was trying to kill his wife. It seemed like a great way to hide a small marital dispute.

With Reddington taking down the president, is this his most successful blacklister he's brought to justice?

Jon Bokenkamp: It's certainly our most high-profile blacklister. In terms of the people in Red's orbit, is this the most powerful? Probably not. I think that he has friends in high and low places and resources in places that one might not suspect. One of the things I'm most proud of is that this story really has its roots in season three where Red went and met then-Senator Diaz and his wife at a ballroom dancing class as they were preparing for his inauguration. So it's a character who we've woven into the fabric of the series for some time. 

Eisendrath: In season six, when this story started, Red had no knowledge nor interest in taking down the president of the United States. He was trying to figure out something entirely different and it turned out that this was what was going on. He needed a way to get out of jail, and he was trying to figure out a way to get leverage to do that and it turned out that this was what it was about. What I like about that is that the story unpacked from a very specific place that was incredibly important to Red. His freedom and his ability to get out from being incarcerated is how the story began.

We have to talk about the season-ending surprise. Reddington goes to Paris to see Katarina Rostova for the first time in years. You've planted seeds all season about Katarina's backstory, but when did you know that this was going to be the cliffhanger for the season that sets up a bigger mystery for next year?

Bokenkamp: We talked about Katarina all season and we've known that platforming her in a more prevalent way is something that we wanted to do. We've met Katarina before in these unusual flashback episodes, so we've seen her played by Lotte Verbeek in her previous form as a younger agent. But now, to see where she is, what she's up to, why she would ever drug and grab and drag away Reddington is the whole reason to come back. 

Eisendrath: This character is central to the mythology of the show. With so many answers that we want to lay out and the stories we want to tell about this woman next season through the eyes of these characters and through Reddington and getting to know her, who she's become, what her history is with Reddington, I think we have a lot of really compelling story to tell. We're sort of leaping into season seven with zest because it's going to be really, really compelling stories.

How different is this Katarina versus the Katarina we've seen in the past?

Eisendrath: When we saw Katarina in the flashbacks, she was a young woman who was almost always traumatized. We were dramatizing these moments of great peril for her in the aftermath of Liz shooting her dad and her trying to figure out how to get free of the Russians or the Cabal, so we've seen her on the defensive, very clever and incredibly crafty. But here, right away, she's on the offensive. She's obviously a mature woman now. She's someone who's much more -- just in what little we've seen of her -- much more prepossessing and more dangerous.

In terms of the actress playing the older version of Katarina, Laila Robbins, what were you looking for in terms of finding the right person to portray her?

Bokenkamp: John and I talked to Laila at great length about the character and who she was and what the role was. What's so great about Laila is that she's both mysterious and formidable, and she is very Reddington-like in a way in that she feels like an enigma. Remember, this is Elizabeth Keen's mother who not only abandoned her, but sent her child away to be raised by somebody else. Why on earth would she do that? Why did Reddington have to hunt to find her? What has she been running from? With this mature adult version of Katarina, we wanted to platform just how mysterious she is filled with both questions and answers. Laila did a great job of that.

At some point, could there be a mother-daughter reunion between Liz and Katarina? Do you have thoughts about what that could look like if it manifests in the new season?

Eisendrath: The thing that's interesting about what you asked is, what would Liz ask? What would Liz what to know? What would Red ask this woman, let alone the entire Task Force? She presents so many questions and in a way it feels like, how does this make any sense? This woman that Red has just told Liz is not a threat, that she's a good person and she should bring her daughter back and everything is OK. Yet he hunts her down, finds her and she does this horrible thing [to Red] in return. What is going on? This specter of who this woman is, is the promise of next season and finally, after six seasons, really getting some answers on this woman and her connection to Raymond Reddington and Elizabeth Keen. 

And remember, Liz has no idea about Katarina Rostova. Red didn't tell her he was going to do this. It's always fun and very enjoyable for the audience, when at the end of the season, one or the other of our two main characters, Red or Liz, are way ahead of the other one. So Red in this case, though he was drugged by her and has been taken away, knows way more than Liz does about Katarina, about her existence, about where she is. We will get to unpack this in a way that is very satisfactory in season seven when and how to let Liz catch up to Red and know that he has interacted with Katarina Rostova. 

Is season seven approaching the endgame for the series?

Eisendrath: We always envisioned sooner or later all the characters that we have talked about and dramatized -- in Katarina's case, in the past -- would come and become present tense in the story. I think there's a lot of story left to tell. I think every season has been incredibly personal and has had huge revelations and next season will be no exception.

The Blacklist returns Fridays at 8 p.m. ET/PT this fall on NBC.

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