Catastrophe Killer: An Arknights Pre-Canon SI

A Janitor's bad day leads him to a world filled with magic, futuristic settings, a world filled with demi-humans who call themselves humanity, and a terminal illness with no cure. While he's not a "human", but rather an alien. Making him safe from attaining the magical cancer rock disease known as Oripathy. Sadly, he cannot use magic, but is able to make it up with a unique training method he used on a whim for his training. Safe to say, it didn't take long for him to become the most sought out Catastrophe Messenger in Terra.

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Meti's Sword Manual from the famous Kill Six Billion Demons comic series.