The idea of "canonicity" in RPGs is an odd one, given the whole point of both videogame and tabletop RPGs is that the players decide what the main characters do. But sequels, spin-offs, and sourcebooks have a habit of choosing particular outcomes as the "real" ones—unless it's the Elder Scrolls, where every ending of Morrowind happened at once.

Baldur's Gate 3 had to deal with characters from Baldur's Gate 2 having canonical events happen to them between games, like Minsc and Boo being turned to stone or Viconia and Saarevok backsliding into evil, and now the cast of Baldur's Gate 3 are in the same boat. The latest Forgotten Realms supplements, Adventures in Faerûn and Heroes of Faerûn, move the timeline on from 1492—when BG3 happens—to 1501, and hint at the fates of its cast.

Quotes from Karlach crop up throughout Adventures in Faerûn and she appears more than once in the art for both books alongside Astarion, Shadowheart, and Minsc. And in none of those illustrations is she a mind flayer or a pile of ash—two of the potential outcomes for her in Baldur's Gate 3.

The best ending for Karlach is that she returns to Avernus with the Infernal Engine still burning away inside her chest, which a lot of players weren't happy about, and in the epilogue Larian added post-launch she mentions finding blueprints for something that could prevent the timebomb in her chest from blowing up. Seeing her run around the streets of Baldur's Gate, fight a remorhaz with Astarion and Shadowheart, and generally be a heroic adventurer who doesn't have a gaping hole in her torso, makes it plain that Karlach's canonical ending is the good one.

Funnily enough, the same pictures depict Shadowheart with dark hair and bangs, which isn't how she looks in her best outcome in BG3, where she dyes her hair white and parts her hair in a way that makes it look like she didn't just go through a messy breakup. But hair color isn't proof that canonically she never gave up on worshipping Shar, and it's entirely possible she just changed her mind about being a silver fox.

Although if you helped Shadowheart achieve her redemption arc it's entirely possible that in act three you told Dame Aylin about the jerk wizard Lorroakan who wants to steal her immortality, and she wrecked the dude as he so richly deserves. According to Adventures in Faerûn that didn't happen, and Lorroakan is still alive and holed up in Ramazith's Tower. However, it's mentioned that he was "Humbled by a recent failed bid to achieve immortality through undisclosed means," so at least he didn't succeed. Presumably the canonical PCs lied to both him and Dame Aylin, preventing the conflict from ever happening.

Astarion gets his own supplement in a digital add-on called Astarion's Book of Hungers, which includes rules for playing a half-vampire dhampir as well as a string of adventure outlines where you get to hang out with everyone's favorite vampire spawn, although these are noted as being set before the events of the videogame. No canon conflict there.

It's not all videogame characters in these books, of course. The heroes of the D&D cartoon make another appearance, just like they did in the movie Honor Among Thieves, looking slightly older and perhaps a bit more competent. Elminster rates a mention too, though he's said to have been missing for a few years, which conveniently explains why he's not around to solve whatever problem your own players are involved in.