Humans live in different planets and unknown to each other. Humans isn't the lone creature. There millions other. Some of them are known to Humans on Earth, some aren't. The known space or universe houses millions of creatures. Several Intellectual beings. Some Powerful beings. Each and everyone have their own goals.  Earth Humans have their own counterpart Grimm Creature and so the other. In an experiment of Grimm underworld dark forces, a breach went off. Unleashing thousands of their experiments to Humanity. These experiments seems to be other Grimm creature. Mindless, blood thirsty and hungry experiments. Humans called them Zombies. At first, with their current technology, humans are gaining the upper hand. However, humans got relax on fighting that they didn't noticed these zombies are evolving/mutating. Gaining more strength, speed and lethality. In a decade, humanity almost lost 40% of their population. 60% of their cities and developed lands.  In a brutal fight, several surviving cities went to ruin. Millions of humans died.  A conclave of Grimm creatures headed by powerful beings such like Vampires and Werewolves, a decision was made. To save humanity from their extinction.