Upgrade System In The Undead Skeleton USUN

In a world where there is no technology and there are ferocious beasts that prey on humans.
Humans also become strong to fight the beasts to survive.
There is a lonely child who wants to become a warrior to survive and get out of poverty.
However...
Plans don't match reality, he dies when he's dealing with a beast that's stronger than himself.
And when he was about to die, a notification popped up.
After that his world began to change...