The weakest magician of instant death and destruction1

Only I know the true ability of 【Instant Death Magic】! Arc was given the off-the-beaten-path skill [Instant Death Magic], which 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 even 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" succeeds, and his "skill" levels up! Skill grows? I've never heard of such a thing..." puzzled Arc. But as a "level 1 instant-death magician," Ark finds himself involved in a major national crisis.

Only I know the true ability of 【Instant Death Magic】! Arc was given the off-the-beaten-path skill [Instant Death Magic], which 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 even 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" succeeds, and his "skill" levels up! Skill grows? I've never heard of such a thing..." puzzled Arc. But as a "level 1 instant-death magician," Ark finds himself involved in a major national crisis.