In the fantasy/classical world where the god gives a system to make the people's lives greater when they turned 15 years old, there is this one guy without a system.  For 3 years that guy was barely living any good.   But everything changed when he finally learned that he had an innate skill that can absorb a monster's skill and make it to himself.  At the same time, something worst had happened to him that was caused by the demon lord, thus he swore to destroy every single monster he will see and slay the demon lord by himself.