Sokushi to Hametsu No Saijaku Majutsu Shi 1 (nanairo comics NC-015)

[Machine Translation] Only I know the true ability of (instant death magic)...! Arc was given a skill (instant death magic) that is said to have an almost 0% success rate, although it is a "must-kill if it hits. Moreover, the upper level limit for growth is only "1", making him the weakest adventurer, and his childhood friend, who had promised to form a party with him, easily gives up on him. In the midst of his despair, he is attacked by a vicious monster, and his (instant death magic), which he unleashes in desperation, succeeds, and his (skill) level is improved! Skill grows? Arc is at a loss, but as a "level 1 instant death magician", he is involved in a great event that will shake the country.

俺だけが知っている【即死魔術】の本当の能力…！！“当たれば必殺”だが成功率ほぼ0％と言われる外れスキル【即死魔術】を授かったアーク。しかも成長性を示すレベル上限はたったの「1」で、最弱冒険者として認定されてしまい、パーティーを組む約束をしていた幼馴染みにもあっさり見限られてしまう。絶望の最中、凶悪なモンスターに襲われ破れかぶれに放った【即死魔術】が成功したことで、【スキル】がレベルアップ！スキルのほうが成長？そんなの聞いたことが…と戸惑うアークだったが、「レベル1の即死魔術師」として、国を揺るがす一大事に巻き込まれていくのだった。