My Monster Fuses Everything

Apocalypse befell, and roommates performed a skyward ritual.
As the fog descended, the Book of Blood Spirit appeared in the hands of all survivors, and only those with a Contract monster could survive.
Su Han directed his gaze to the neighbor who had already mutated and twisted, silently pulling out a gleaming fruit knife...
Grotesque meat corpses, giant spiders with gaping mouths, eyeballs enveloping the city, one-eyed sharp-toothed hand monsters...
Civilization fell, cities shattered, and all humans became meat for the monsters. There was only one way to survive:
Use monsters to fight monsters.