Story Idea (Exmortis Crossover)

I am unsure if this is a good idea, but have you ever heard of the point and click horror game Exmortis? If no, play it at your own risk.

Anyway, the idea is one of the mane 6 (Twilight or AJ) is travelling an apocalyptic Equestria as a lone survivor after the attack of the hordes of the Exmortis (a paranormal legion of zombie like demons that signify their arrival with a fast moving red cloud that blackens all under it). The story will be told in a first person point of view from the chosen character and their experience travelling.

Is it a good idea and should it be a one-shot or multi-chaptered?