MTL - Deviant Immortal of Uncanny Dao - Chapter 17 found it

Riven was nothing but a powerless runt until necromancers razed his village and butchered his sister before his eyes. As his blood soaked the dirt, his Sigil ignited, granting him a gift. He can die and reform, a cycle that resets with every rank he climbs.But fate wasn’t done screwing him.In the catacombs beneath the ruins, he found the Death Ledger—a death contract that bound him against his will to hunt those who’ve cheated death across dimensions. Historical legendary heroes and villains.From the noble King Arthur to the conqueror Genghis Khan and the fabled king of Uruk, Gilgamesh, together with Ivan the Terrible and Vlad the Impaler.Every mission is a gamble. Fail once, and the Ledger brands him a death-cheater, turning him into prey for the same immortal fugitives he hunts. And the reward? He gets to keep them as his undead summons.His enemies will not stop coming. They want the Ledger, the Sigil, his power. To survive, he will wield death itself, growing stronger with every soul he claims, because in a world this savage, only the relentless rise.[-Congratulations, you have successfully killed Alexander the Great and claimed him as your summon...you can start on the path of evolution-] The words, scrawled in blood, burned across the page of the Death Ledger.

Lynn traveled to a magical world similar to the European Middle Ages but possessing magical powers.

Here, magic can affect and control elementary particles.

So…a scientific theory is a magic formula!

Wizards called spellcasters who are actually scientists, under the siege of the Holy See, struggled to explore the truth of the world, until the arrival of Lin En, which set off a renaissance of an alien version…

(PS: The 2 million-word work “Hogwarts Bloodline Wizards” has already been completed, and the quality is guaranteed!)

- Description from novelbuddy

When a fantasy kingdom needs heroes, they skip the high schoolers and summon hardened Delta Force operators.Lieutenant Cole Mercer and his team are no strangers to sacrifice. After all, what are four men compared to millions of lives saved from a nuclear disaster? But as they make their last stand against insurgents, they’re unexpectedly pulled into another world—one on the brink of a demonic incursion.Thrust into Tenria's realm of magic and steam engines, Cole discovers a power beyond anything he'd imagined: magic—a way to finally win without sacrifice, a power fantasy made real by ancient mana and perfected by modern science.But his new world might not be so different from the old one, and the stakes remain the same: there are people who depend on him more than ever; people he might not be able to save. Cole and his team are but men, facing unimaginable odds. Even so, they may yet prove history's truth: that, at their core, the greatest heroes are always just human.

I finally managed to kill the demon lord.