Author note -If anyone didn't get the obvious hint, or forgot this story, I'd remind you clearly, this Amelia flashback is happening simultaneously when Rio is taking part in the tournament and preparing for the dungeon event exam. (So whatever happens now is after the event of ch346 and before the opening scene of ch347)
_Meaning after his plan to kill the students failed, and before he entered the dungeon for the final exam where he got trapped.
__ personally I'd suggest you re-read chapter 346 for better understanding and immersion.
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Rio who was lying in the academy's nursing room, resting in bed while waiting for the effect of Nyx's curse to be over. (Given in ch346)
Since he couldn't leave the bed and the academy was busy collecting the corpses of students who died in the attacks of Evil's Scion (Eclipse) _ he just stayed there, thinking about how his plan failed and everything he worked for a year turned to ashes.