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Thor the God of Thunder had decided to descend. In fact, the Aesir Gods led by Odin, the God of Light, had been keeping a close eye at Maria since she started her journey. It wasn't abnormal for ghosts to emerge, nor for them to grow strong and become monsters and threats to humanity. But a ghost that could retain her memories, sanity, and even gain insane amounts of skills, abusing the Soul Book and even utilizing completely new Unique Skills? Now that was dangerous.
The existence of Undead in the surface, the world of the living, was a thing that had only begun happening a few thousands of years ago, when the power of the world destabilized due to the Evil God's Fragments engulfing everything with miasma and chaos. Although Odin agreed to defeat his own son due to being a threat to his reign, he never expected Loki would utilize this opportunity to spread Chaos across the world even further.
Check "The Protagnist's Sister Is Actually the Strongest"!