The System gave everyone on Earth a Class. It gave Marcus an error message. When the Integration hits, paramedic Marcus Hale is mid-shift, elbow-deep in someone else's emergency, which is exactly where he has spent his whole life. Everyone around him gets stats, skills, and a shiny new Class. Marcus gets one word: NULL. No Class. No skill tree. Flagged for deletion. But an error the System cannot read is an error the System cannot limit. Because Marcus has one trait no Class was ever offered: kill it, and take what it could do. Its blink. Its blade. Its voice. Now the monsters have his scent, a white thing with a pen is unwriting his city one wall at a time, a warlord is building a ladder out of desperate survivors, and every deadline ends at the same crater. They call him an error. They should have read the label twice. Zero Day is a complete novel and the explosive first book of the Errorborn saga: a LitRPG apocalypse featuring a competent, sharp-tongued hero who refuses to stop doing his job at the end of the world, hard-won progression with real costs, a dungeon that would rather be a hospital, found family that fights like a machine, and a System that has started leaving him personal notes.