Lizzie becoming a Heretic in Legacies theoretically stops the Merge from happening but as Vampire Diaries showed, it will bring brand-new problems.

After more than three seasons, Legacies finally solved the problem of Lizzie and Josie's Merge with a loophole first introduced in The Vampire Diaries. As Gemini twins, the issue of the inevitable Merge, in which one twin would absorb the other, has been hanging over the heads of Josie (Kaylee Bryant) and Lizzie (Jenny Boyd) for years. It's been intermittently handled on Legacies, sometimes a major focus and sometimes being all but forgotten, just lingering as an unresolved plotline. But with Kaylee Bryant exiting as a series regular, it was a story Legacies needed to resolve, and to do it, they turned to The Vampire Diaries.

A common audience complaint for the first three seasons was that Legacies seemingly forgot it's part of the Vampire Diaries universe. Elements and recurring themes that were core components of both Vampire Diaries and The Originals have largely been nonexistent in Legacies. But in the past handful of episodes (initially meant to be the first episodes of season 4), Legacies has suddenly remembered it's part of that universe and, as such, has been bringing back characters from the other two shows for cameos. More importantly, it's started to incorporate some of the established mythology regarding supernatural creatures and magic.

Legacies season 4, episode 9, "I Can't Be the One to Stop You," continued that trend. The midseason hiatus finally solved the problem of the Merge, with Lizzie being killed by Hope (Danielle Rose Russell) but resurrecting as a vampire. According to the established supernatural rules of the Vampire Diaries' universe, theoretically, since Lizzie died and severed her connection to Josie, the Merge is no longer necessary. But while people dying and coming back as vampires was a regular occurrence on the other two shows, Lizzie is a special case. She and Josie are Siphoners, already considered abominations by witches. Because of that, Lizzie can still be a witch and use magic, even now that she's a vampire. Siphoners are the only witches able to do this, so these vampire-witch hybrids were first introduced in The Vampire Diaries as Heretics. As with all magic, Lizzie becoming a Heretic is a loophole, and it's one that could have serious ramifications on Legacies.

Unfortunately, this plot twist underscores the problem with Legacies having been so disconnected from the previous two shows. The concept of Siphoners has barely been explored, so it may not be clear to audiences who haven't watched the other shows why Lizzie becoming a vampire and Heretic is important. Siphoners are rare witches who are born without the ability to create magic of their own; instead, they pull magic from other sources. As Siphoner witch-vampire hybrids, Heretics can technically tap into their own supernatural vampirism for a magical boost or if no other source of magic is handy. With their special powers, Siphoners can do things that other witches can't; for example, they're able to siphon the magic out of werewolf venom, curing vampires from an otherwise fatal bite. Previously, it was only Klaus Mikaelson's (Joseph Morgan) blood that could cure a werewolf bite. Heretics can even cure themselves, tapping into their vampiric magic to siphon the werewolf toxin out of their system.

Currently, there is only one other known Heretic in the world: Valerie Tulle (Elizabeth Blackmore), who was a recurring character in The Vampire Diaries season 7. She was one of the rare TVD recurring characters who left the show alive, if not unscathed, and here whereabouts since then have been unknown. With Legacies bringing back so many previous characters from The Vampire Diaries and The Originals, it's possible Valerie could return, as well, especially since so little is known about Heretics. Lizzie already has to navigate being a newly-turned vampire, and, as someone who already had challenges with emotional and mental stability, the heightened emotions of vampirism will be hard to control. Combining that with her ability to siphon, and Lizzie could go down a very dark path if she doesn't have someone to guide her. It would be a great time for Valerie to return in Legacies season 4 to help Lizzie. Regardless, it will be interesting to see where Lizzie's story goes now.

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Legacies season 4 will return on Thursday, January 27 on the CW.