Eternally Regressing Knight - Chapter 3 - A Day

Gu Ting Yu is a veterinarian with a fear of animals. Haunted by the same nightmare all these years, he is desperate to find out the reason behind those dreams. He then travels into a fantasy realm by fate, entering the Palace of the Twelve Sacred Beasts.

Avo was born to destroy New Vultun. In a twist of fate, he’ll become its only hope for salvation. The gods were the enemy. The slaver. The chains. With dreams of paradise, humanity slaughtered them and stole their powers, opening the path to become more – to ascend. Instead, they nearly destroyed the world. Vast hive cities serve as the final bastions against the crumbling of reality. New Vultun is the greatest of them all, and within its borders, ascenders wage war to determine the laws of existence itself. The world descends ever deeper into ruination. Avo was born a weapon, a slave to his instincts and his masters. He and his brothers were made to burn New Vultun to ashes. Everything changes when Avo gains immortality and becomes heir to an impossible legacy. He must ascend the megacity of New Vultun. He must challenge the very tyrants that usurped the gods. He must finally free the world from chains.

This is a story about the protagonist getting the system, going crazy in the biochemical apocalypse, producing aircraft and tanks, destroying zombies, saving good people and killing bad people.

- Description from novelbuddy

The empire has turned into the land of the undead due to a spell gone wrong. God summoned heroes from countless worlds to purify the empire and plant new hope. Lee Shin Woo, an ordinary earthling, was also summoned. As an undead, that is.