Warning: spoilers ahead for Episode 16 of Arrow Season 6, called "The Thanatos Guild."
Team Arrow has gone through a bunch of changes in Season 6, and Arrow brought members in and out. "The Thanatos Guild" brought none other than former assassin Nyssa al Ghul back into the mix for an unexpected reason, and what she had to say resulted in a big departure from Team Arrow. Thea decided that the time had come to leave Star City behind and focus on a larger--and potentially much more dangerous--mission alongside Nyssa.
Nyssa turned up after she discovered that Thea was being targeted by assassins due to her status as the daughter of Malcolm Merlyn. She teamed with Team Arrow to recover a box containing a map that Malcolm had left Thea. Although it seemed at first that the map was actually a blank piece of parchment, the truth became clear after Thea dripped blood on it. The map revealed three sets of ley lines, which meant that Malcolm had discovered the locations of three Lazarus Pits. Nyssa, determined to right her father's wrongs, took the map and intended to leave all on her own.
Thea was determined to make up for her own father's wrongs as well, and she decided that she needed to go with Nyssa and assist her on her mission. She left Team Arrow with Nyssa (who finally annulled her League of Assassins marriage to Oliver), fortunately alive and well. Roy also joined Thea and Nyssa, unwilling to separate from Thea ever again. The bad news is that fans will lose Thea from the show; the good news is that perhaps all three of these characters will return. Thea surely won't be gone forever, and she could very well bring Nyssa and/or Roy with her.
Of course, depending on your point of view, there may be more bad news to come out of Nyssa's return and Thea's departure. The manner of Thea's departure with Nyssa means that the League of Assassins storyline is still not finished despite seemingly coming to an end a few times. If you're not a fan of that plot, you probably weren't happy to lose Thea to it.
Personally, I'm always happy to see Nyssa, and if the League of Assassins plot has to continue for Nyssa to appear, so be it. My problem with "The Thanatos Guild" is that it brought Lazarus Pits back into the mix. Arrow--and the other superhero shows of the Arrow-verse, for that matter--have a problem with not keeping characters dead. The existence of Lazarus Pits in the Arrow-verse means that deaths feel less impactful because we know that they might not be forever. The stakes are simply lower in life-or-death scenarios. Lazarus Pits can turn into get-out-of-death-free plot devices if not handled properly; I only hope that Arrow doesn't overuse them. Who knows? Maybe Nyssa will destroy these newly-discovered Lazarus Pits as she did the Pit at Nanda Parbat.
Only time will tell. Perhaps Nyssa and Thea's departure isn't really a sign that Arrow plans to revisit Lazarus Pits as part of the narrative and only brought them up because Thea needed a very important reason to give up her potential happily-ever-after with Roy. New episodes of Arrow air on Thursdays at 9 p.m. ET on The CW. No news has been announced about whether or not the show will score a renewal for Season 7, but you can find the fates of other shows on our rundown of TV renewals and cancellations. Don't forget to check out our midseason TV premiere schedule and summer TV premiere schedule for your viewing options.