At Freedom's End Lie Shackles and Fate

Several hundred years ago, humanity’s enemies suddenly appeared. They were enormous, possessed extraordinary regenerative abilities, and preyed on humans. They came to be known as “Variants.” Facing the threat of extinction, the survivors had no choice but to flee underground and build a subterranean city. They constructed three defensive fortresses to protect it, sealing off the entrances and stationing large numbers of soldiers there. Two additional steel fortresses were built inside the underground city. The people lived peacefully in this isolated environment for nearly a century. The protagonist’s parents died when he was young, and he lived quietly with his older brother and sister in the outer slums. Then, in the year he turned twelve, a “giant Variant” suddenly appeared and smashed through the city gates with overwhelming force. The ferocious Variants invaded the outer district of the underground city to hunt humans, and the protagonist watched many people, including his sister, get eaten. He survived with indescribable hatred for the Variants and the pain of losing his family, making his way into the second fortress. One day, driven by his determination to survive and avenge his loved ones, the protagonist enlisted in the army. Everyone in the training camp had an iron will, because each of them had a goal. Three years later, just as their training ended, the Variants launched a sudden attack. Though he had seen firsthand how terrifying they were, he bravely challenged the Variants. After surviving, he joined the expeditionary force formed to exterminate them, and the story continues from there...