Toaru Majutsu no Index: New Testament - Volume 18, Chapter 2: To Where Should You Flee? - Escape_to_Central.

An ordinary student accidentally traveled into an other universe and became the king of a small kingdom. He had gotten a “Cheating Code” of life and was able to complete something extraordinary. This is the story about a king that was able to conquer continents and leave behind him an ultimate legend that will be recited between every generation to come.

- Description from novelupdates

The nuclear holocaust which caused the collapse of the Old Times on Earth should have wiped out all human life on the planet. Yes, the gods set up their beautiful Elysiums to provide sanctuaries for their chosen, but by all rights everyone outside the elysian lands should’ve perished long ago. Yet somehow, human life still managed to persist, even in the deadly, mutant-infested wastelands.

Cloudhawk was a young scavenger who dreamed of being as free as the hawks in the skies, yet seemed destined to live out his life scrounging for scraps in the wasteland ruins. Fate, however, is ever-fickle. A chance meeting with a ragtag group of mercenaries changed the trajectory of his life, bringing him into a world with mutants and metahumans, demonhunters and godslayers, and even gods and demons. Cloudhawk would find his own place in a world that was far greater than he had imagined, find his own path between the zealous light of Sumeru and the whispering darkness of the Abyss… and one day, he would find that even gods may fall.

Second Awakening? A System? Having lost the battle of the Dark Catastrophe to the demons—spawns of the Dark Gods— humanity is given a chance at making things right once again. Born as a twin into a family of high ranking talent awakeners, Damien Terrace is met with trouble when it is his turn to perform the ritual. After awakening the E-Rank Summoner talent, Damien loses everything to his twin. Exiled by his father to perish away from them, Damien goes through a second awakening that changes the play field for him. Ding! [The God of Summons is furious and has blessed you with a system!] [You have fused with the Mythics Summoning System—A system that only allows you to summon mythical beasts!] [Obtain nineteen Magic Essence Cores to boost your core to the next level!] [Slay 200 more demons to unlock another summon!] Damien’s only option is to evolve!

“Classes” are given at the age of 10, and the presence or absence of “skills” greatly affect life. Arel, the son of “Sword Princess” Fara and “Magic King” Leon, has been branded as “Classless”… But even without a job or skills, Arel believes he can persevere through effort.