MTL - The Right Way to Rise Uchiha - Chapter 494 New Era

Xia Yan passed through to the world of Bleach and became an eleven-year-old boy. He was rescued by Renji and Rukia in the Seireitei, and he was studying at Shin’ō Academy, and he was a student of the same class.
He is a top student of the Shin’ō Academy, and he is vying for strength by various teams. In the end, he has entered the first team of secret mobile maneuvers.
He will use punishment to kill every person who tries to threaten the martyrdom and protect his loved one.

- Description from Novelupdates

Three centuries ago, reality tore open and parasites from another dimension poured through. They didn't just kill humanity—they consumed it, wearing human faces while spreading like a plague through every major city.The strong became Originals, commanding lesser generations in a hierarchy of monsters that humanity calls Bloodsuckers.Now they own everything. Governments, corporations, entire cities bow to creatures that should be myths.That's when the Shadowguard was born. They were humanity's elite hunters equiped with the skills and tools to put an end to the bloodsuckers. But they became nothing more than political tools, protecting the wealthy while the poor got harvested by Bloodsuckers like cattle.Kaine Cross was one of the best hunters alive until he walked away from the corruption. He decided to become a solo hunter.Unfortunately, going freelance got him killed by the very monsters he'd spent twelve years learning to destroy.Death was supposed to be final. Instead, Kaine awakens in an ancient tomb with a system that makes him something the bloodsuckers have never faced—a predator that grows stronger with every kill, commands armies of the dead, and turns their own kind against them.Every kill feeds his strength. Every corpse becomes a loyal soldier. Every battle expands his growing army until even vampire lords will bow or join his ranks permanently.[LEGENDARY WEAPON ACQUIRED: SOUL END - THE DEATH REAPER][SKILL UNLOCKED: CORPSE DOMINATION - UNLIMITED ARMY CAPACITY][MORTAL ESSENCE SYSTEM: ABSORB POWER FROM EVERY KILL]Zombies. Liches. Reapers. Wights. Revenants. And even monster girls; beautiful, deadly creatures once enslaved by the vampires now serve at his side, bound by Kaine’s will or by lust, vengeance, and freedom.He's not a hero. Heroes save people. Kaine Cross collects enemies and turns them into allies, whether they want to be or not.The vampires ruled through fear for centuries. Now they're about to learn what happens when death itself holds a grudge.Join Kaine as he builds an unstoppable army of undead and monster girls, and leads them on a path of blood-soaked revenge.Build an army. Claim your revenge. Rule the dead.

“I can definitely become an immortal,” she said.He looked her up and down, sneered, “You can indeed become an immortal, a Lazy Immortal.”She breathed a sigh of relief, fully self-assured, “A Lazy Immortal is still an immortal.”He was speechless. Great, very powerful, who says a lazy person can't become an immortal~

How long will it take for a murdered thief to distill into something monstrous? In this story the darkness only grows stronger, and its reach only grows wider as it expands by devouring the life that surrounds it. This is a dark fantasy where evil is a where, not just a who that focuses on slow progression, undeath, and atmospheric storytelling.