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Everything happened out of nowhere. Sylph managed to use her powers to see through the future two seconds, exhausting her eyes in the process several times to barely save the life of a few, such as the Lord of the Fiefdom and Mary. After the Dungeon Break completely destroyed the manor where everybody was located, Sylph's group split. Her father Allan and Shade and Nepheline, Aquarina's parents, decided to go on their own to clean the entire place from monsters.
For some strange reason, Dungeon Breaks were happening even where there were no dungeons nearby. Monsters began pouring in like crazy, and people was growing desperate. Many nobles were getting killed in the process, and Sylph's father alongside Aquarina's parents were barely able to stop the monsters from killing half the population of the Noble District, but even with their speed, what they could do was limited at the end.
Check "The Protagnist's Sister Is Actually the Strongest"!