Upgrade System In The Undead Skeleton

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...