The System is here! Most of the population of the planet were suddenly turned into zombies, while the rest acquired new abilities. Abilities that let them run from the zombies, save themselves, hide, and try to survive. But it only takes one bite that almost always means death.
Our hero was standing in line when the apocalypse came. He was lucky at first to be caught in a crowd when the zombies tried to bite him on the leg. Really lucky, because those trials unlocked new abilities for him. He got to be one of the first to choose his character class. He had to do it quickly, before someone else got the best one . . .
Considering he was introverted, maybe even a little antisocial, the choice was easy. Master of Shadows? Nope, already taken. Okay, Master of Darkness! Also taken.
Okay, Ghost class also works. And it is a gold class, even though the bonuses you get are a little . . . questionable.