Call of the Dead: from Class F to Lord of the Underworld

[With talent solidified, F level is the end point? Sorry, I'm an exception. Awakening at the age of 12, Shen Yuan was rated as the most useless F-level undead summoner in the continent, and could only summon a lame skeleton dog. Everyone said that his life was over, even his best brother was an A-level genius. How harsh the ridicule in the past was, how loud the slap in the future will be! But only Shen Yuan knows that his talent automatically evolves every 10 levels! From F level to E level, the skeleton is shaped; from D level to S level, the bone dragon comes! When the so-called god-level man knelt down and begged for his life, hundreds of millions of undead souls behind Shen Yuan covered the sky and the sun. In this life, he will overturn this damn temple of talent and personally break this iron law that has lasted for thousands of years! (PS: There is a reward posted all over the Internet. Guess what my skeleton can evolve into in the end? See you in the comment section!) [Also known as: "Talent cannot be changed, I am an exception", "From F level to the king of the underworld"]