If I remembered it right (sorry I read the novel like more than 10 months ago so sorry if its wrong) SPOILER BELOW
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There's like bloody massacre in the school. Monsters came out from the gate and kill the students. The shadow in his sister is on the same level with some monsters that came out. Luckily mc managed to go there and save his sister.
I'm really sorry if I'm wrong cuz it's been ages of my last reading of the novel ( ̄∇ ̄")
Spoiler ahead!
The novel is quite enjoyable so I do recommend it for anyone in their spare time.
https://boxnovel.com/novel/i-alone-level-up/
my pea sized attention span started skipping the ending a lot, but at least its completed with a few extra side stories.
Im actually excited for this arc because its cool. No worries, no one of importance dies. The monsters attack the school that his sister is in, but she has a strong shadow attached that protects her and her class. He gets the message and puts his shadows to clear the dungeon by themselves (lol) and says I gotta go. either his showdown exchange cool down JUST finished OR he rode kaisel I cant recall, to the school and shows up all BAMF. The monster that appears is a foreshadowing to an underlying giant plot that happens near the end. Also everyone records him being awesome.

Isn't there a time limit for when he can use shadow exchange again? I feel like he just wasted it. What if he doesn't make it back in time if his sister's in trouble.