Qual was one of the Demon King's top-tier commanders, and eighty years back he was considered such a massive threat that the Hero's Party had no choice but to seal him away. He was genuinely that powerful.
What really made him dangerous was his invention of Killing Magic, also called Soul Track—a piercing spell that could cut straight through magical defenses and equipment enchanted with magic resistance. The thing was unstoppable; it didn't just break through barriers, it could obliterate the target's physical body entirely. During his rampage, Qual's magic was responsible for wiping out roughly 40% of the region's adventurers and 70% of its sorcerers, which gives you a sense of just how lethal it was.
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