Attack on Titan's WHAT IF : Zombies

"When there's no more room in Hell, the dead will walk the Earth."

Summary :

6 years after the death of Eren Yeager and stopping the rumbling, humanity is slowly repairing itself from the aftermath of the chaos. What if it was just a delay as a preparation for something worst? Unbeknowst to them, a new unknown threat rises to the grave to threaten humanity once more. Will they survive or perish?