Qual

Qual was a high ranking demon in the Demon King's army. 80 years ago, he was so powerful that the Hero's Party was forced to seal him away.

Qual is the inventor of a powerful piercing magic called Killing Magic/Soul Track. This magic is able to pierce magical defences, as well as equipment imbued with magical resistance. It was so strong that it was able to eradicate the target's body itself, as well as being responsible for slaughtering 40% of the adventurers and 70% of the sorcerers in that region.

[Spoiler] After the Demon King's death, Himmel would visit the village every year to inspect the seal. Almost 30 years after his death, Frieren and Fern came to the village as the seal's timer was reaching its end. Qual was killed by Frieren. [/Spoiler]

[Spoiler] Killing Magic/Soul Track was analyzed and improved by the humans with the help of Frieren to slay demons. It's considered a regular tier spell 80 years later. [/Spoiler]