Spoilers ahead for the December 6 episode of Legacies on The CW.
Caroline Forbes has been conspicuously absent from Legacies' first season so far. Alaric and their daughters have been plenty busy at the Salvatore School for the Young and Gifted, and newcomers to the Vampire Diaries franchise may not have spent much time wondering about where Josie and Lizzie's mom is. Fortunately, the latest episode finally revealed what Caroline is up to: she's searching for a way to prevent one of her daughters from dying in six short years as part of the witch ritual known as The Merge.
Showrunner Julie Plec explained why Legacies revealed what Caroline is up to, saying this:
We don't want Caroline to be an irresponsible mother. She's a great mother, and she's always been a great mother to these girls, so if she's not going to be there for this huge event in their lives, it damn well better be for a good reason. That was one of the reasons we wanted to introduce this secret that she and Ric are keeping from the girls.
The Caroline that Vampire Diaries knew from the original series wasn't a person who would just abandon her family, so even Alaric's claim that she was off on a "recruiting mission" felt kind of thin. She was utterly devoted to her daughters on The Vampire Diaries; surely it would take something massive to pull her away from them long enough that she'd miss their sweet 16.
By revealing Caroline's real mission to prevent The Merge from happening to her daughters, Legacies gives an understandable explanation for her absence and leaves the door wide open for Caroline to appear, if Candice King is willing and able to reprise her role. Lizzie and Josie only have until they hit 22 before they'll face the Gemini Coven Merge for twins.
The Merge would see the twins merged together with all of their combined magic absorbed into one twin. The stronger twin that absorbed the power would survive the ritual while the one drained of power dies. Obviously Caroline would want to prevent such a thing from happening to her girls, and this imperative mission explains why Alaric hasn't been constantly frustrated by her absence. Why wouldn't he support her in finding a way to save their daughters?
Julie Plec also explained in her chat with TVLine what The Merge means for viewers:
Well, what's fun about that little nugget is that someone who only watches Legacies and didn't watch The Vampire Diaries probably has no idea what we're talking about. So they can take it upon themselves to go back and learn, or they can keep watching it and have it play out for them in real time. It will be a secret that Ric will keep close to his vest all season, and it's information that will be fighting to get out. I think it will be fairly game changing for the girls when they find out what Jo was talking about in that moment.
For the majority of Legacies so far, viewers haven't really needed a working knowledge of all things Vampire Diaries and The Originals (which even the cast doesn't have), so anybody wanting some early info on what The Merge means for the twins will have to do some digging. Alternately, they can just wait and find out what's in store for the twins if Caroline isn't successful in her off-screen mission.
For now, we can probably count on watching Alaric continue to keep this secret from his daughters, and it may become harder as they continue to question where there mom is. Missing their sweet 16 may not be something they just forget overnight. They got a tease that there's more to them than what they know courtesy of their temporarily-resurrected biological mom in this episode. Could that be the spark that lights a flame of curiosity?