From what im understanding they were sucked into an abyss like dungeon, the environment/atmosphere of the dungeon corrupts everything, including ppl and objects, turning them into monsters and the overflow is like a flood of the corrupting gas/fog concentrated so it seemingly just destroys things instead which is why the mosters also get pulled in.
I use the word fog/gas in this context because judging by how they retreated to underground the overflow doesnt operate the way a water flood does since he said the subway was safe from it. So its probably like a fog thats aggressive and can kill/corrupt.
Thats is what ive gathered, also judging by the ML hesitation when he answered how he knew what overflow was its a safe bet to assume time works differently in the abyss as well, thats always the case for these 'hunterverse' manhwas(which thats what this seems to be, not a guideverse). Sorry for the yapfest lol. Maybe this will be helpful tho
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So they are stuck in the abyss, and in the abyss there are different zones. There’s the jungle, the reed sea, the Japanese zone, salt take etc. The overflow was called the flood in the novel. The flood or overflow whatever you wanna call it, will continue to chase after them (especially yohan) and is the main “villain” in this arc. The ml has been in touch with the flood so much that he has become corrupted. Being corrupted makes you lose your sanity, you slowly feel less human and more monster. Which is why the ml is currently trying to escape with mc from the flood, too escape to another zone where the flood isn’t active. (It takes a while for the flood to travel from one space to another)
I think I explained it really bad, sorry if u don’t understand. If you want more spoilers I can explain it better
I really like this but I kinda don’t understand the world building at first I thought it was an esper guide verse and that they were sucked into a dungeon gate but idk what this overflow is