spoiler
the black portal that shows up at the end of the great dungeon didn’t actually disappear, but instead appeared with the exit portal. only gaia skill holders were able to enter it, and they were given the option to either fight all the monsters inside or sacrifice themselves to give earth another chance at the dungeon in 10 years. at that time, there was only kangjin and seo yoon with gaia skills, but they were obviously not in any state to fight. they both realized this, but seo yoon quickly said he’d sacrifice himself in the black portal so that everyone else could get out, but kangjin didn’t want that to happen so he used gaia’s mind (which he had already copied earlier) and changed seo yoon’s memories to the ones we see when he thinks of the great dungeon. obviously, after everyone left the great dungeon, kangjin entered the black portal and chose to sacrifice himself.
Spoiler----
In the novel, I really thought Kangjin is a bad person but turns out I was wrong. He seemed like a villain at first but that's just because he manipulated Yoonseo's memories to ease the burden of the truth. This made me feel even more sad by his death when I found out what he really did. By the end of the great dungeon arc, I was still hoping he would make a grand return but it didn't happen as it was later confirmed that he really died in that dungeon
Spoiler
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His friend Domin pretty much lost his mind completely and tried to kill his team mates to put them out of their misery, Kangjin ended up having to kill him just as Yoon Seo got back to the base. Kangjin copied Domins power and used it to change Yoon Seo's memories. Turns out Domins other power Gaia's dream allowed him to go into a trance like state that looked like death, Yoon Seo knew but didn't say anything not wanting him to be killed, but because his memory was altered he forgot and Domin was still in the dungeon when it closed. Everyone agreed to say Domin was a hero and sacrificed himself etc so it matched what Yoon would remember.
IDK quite why, maybe bc of Yoon Seo's rank but it was like he still felt the same way he would have knowing everything that happened, but he didn't really understand why. It's why he always got so angry when people talked about Domin 'dying for their sakes' and made it pretty damn clear he had no intention of staying alive after the dungeon was cleared after losing him. I think he felt the guilt of leaving Domin in there when he was still alive as well but bc of his altered memories he thought it was because Domin died and he wasn't there to protect him.

novel reader, please tell me what happened to Kangjin? (/TДT)/