Spoilers ahead for the latest episode of Star Trek: Discovery, called "Saints of Imperfection."
Star Trek: Discovery Season 2 has been all about character development, as everyone from Ash Tyler to Saru are switching roles and changing in big ways. Now, another character has changed in the supposed conclusion of a Season 1 arc that took a really interesting turn in Season 2. Sick Bay may want to clear a space on its staff, because Lt. Commander Hugh Culber is back and very much alive.
So, how in the name of Starfleet did this happen? Basically, when Stamets found his partner dead, he kissed Culber and some of his DNA transferred into his body. Thanks to the Law of Thermodynamics and the fact that Stamets is tapped into the Mycelial Network, Culber's DNA was transferred into the network, and reconstructed within the space.
Unfortunately, this had some negative side-effects on the Mycelial Network. Culber was essentially treated as a foreign invader, and had to drench himself in the branches of a poisonous tree to prevent being harmed by the spores. This prompted May, the ghost girl Tilly was seeing that was actually a sentient alien from the Network, to kidnap the Ensign and beg her to "kill the monster." The monster was a very scared and disheveled Culber, seemingly out of his mind.
Once Stamets (who had entered the Network with Burnham to search for Tilly) was able to figure out what happened. He realized all would be well if Culber would leave. Culber couldn't cross from the way Burnham and Stamets entered though, which complicated things greatly. Luckily, there was still the cocoon that May used to transfer Tilly over, which could be used in reverse. May agreed to sever her link between worlds, and Culber emerged from the spore mass reborn.
Culber is back, but it feels like this Star Trek: Discovery resurrection won't be so clean. He'll undoubtedly have some trauma from his time in the Mycelial Network, and did anyone catch that the man who killed him is now back on board the Discovery? What will the Lt. Commander do when he rounds a corner and comes face to face with Ash Tyler? I'm guessing if he pulls his phaser, it won't be set to stun.
It's certainly going to make Ash's time on Discovery more complicated, as the split Human/Klingon already had eyes on him to begin with. Granted, he has no reason to hurt anyone now that his secret is out, but it's not like Culber has been up on all the developments ever since Ash snapped his neck. Will the two make amends, or just try to ensure they stay on separate sides of the ship until he goes back to Section 31?