If you don't mind spoilers!
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Yohan's power is Purification and he nullifies the effects of the Abyss and purifies the objects and people inflicted with it. He's also the first purifier in the story (the people who manifestes with newfound ability are the only ones eligible to be S-rank, making Yohan the S-rank Purifier and those who manifests as purifier after him can only be A-rank or lower). The halo at Yohan's back is the physycal manifrstation of his power.
Abyss is basically hell them because you instant go cray-cray and turn into monster once inside, foods spoil or becomes outright inedible, and letters went illegible, but the purification counters it. There are subtle hints about Yohan's purification manifesting, like he wasn't affected by the Abyss even though he was trapped there for a long time, the foods he touched became edible, and Hyun-mook regains his consciousness earlier than expected when he was with Yohan (and Hyun-mook became normal for a while when he consumed Yohan's blood).
This is from what I can remember, I apologize if it's too lengthy
The train was actually a monster capable of making an illusion to lure prey and they decided to fight it since it keeps pursuing them. The monster, coupled with the Flood, made Hyun-mook suffer heavy injuries. He defeated the monster but he was closed to dying. When the monsters around smelled the blood from the train monster, Yohan unconsciously released his power, creating a pillar of light and a purification field. The swarming monsters were purified too and Hyun-mook, suffering from slow regenation due to Abyss, was also healed

I'm so excited for the next chapters! I really love the moment where Yohan's powers would awaken and save Hyun-mook!