Only I Know About "Leveling Up" in This World

In a future where a person’s worth is determined by their Level, Theo alone remembers how to gain levels. Once an S-Rank adventurer, he is reincarnated a thousand years later, when people have forgotten the power of “Level Up.” Everyone is born at Level 1, and humanity is regarded as the weakest race, living in fear of monsters. Chosen as a sacrifice to the monsters, Theo knows that humans can become the strongest because they alone have the ability to Level Up.