The story takes place in an alternate universe sometime in the mid-season 3 period.
Twilit Spargle drugs her friends with a solution of her own creation and convinces them through the potion to help her overthrow Princess Kelestia and Princess Luny. The five drugged up, enchanted ponies agree and Twilit teleports them to Canterlot. Realising that if it was this easy to teleport through an allegedly foolproof magical barrier then it should be easy to teleport straight into the princesses' room, Twilit teleports them again and they capture the princesses. Twilit uses a spell she created herself over years of plotting and extracts the magic from the princesses, translating their alicorn hood to her. Twilit becomes a super alicorn and prepares to rule over the land. However, the effects of the drugs make her friends go insane and adopt Zombie-like attributes. Twilit flies away into an unspecified jungle where she meets Rainbow Basher, who, it turns out, was not affected by the drugs as much because she "isn't a total wimp". The two team up to fight the Zambies and try to save Equestria, but Rainbow soon finds she has misplaced her ApplePhone and must find it for it is vital for her to be able to contact help.
Zambies is a work in progress story and there have only been 4 chapters so far.
At the start of the story, Twilit show disdain for her friends, especially Vanity and Rainbow Basher. However, as the story moves along, Twilit grows closer to Basher in a platonic way, realising that despite her many controversial views and morals, she can still do good for the community and can still care for others.
Rainbow Basher has only a marginally different colour pallet to that of Rainbow Dash. The rainbow stripes in her hair are visibly brighter and more saturated to correspond more with the Rainbow Flag for the sake of irony. Her cutie mark sets her apart from Rainbow Dash in that it's the symbol of the Nazi Party.
In the story, it mentions several times that Rainbow Basher is homophobic. In the second chapter, she mentions how she was intending to go to a gay pride rally before they were teleported to Canterlot. At first everyone is confused because they all know Basher to be very anti-LGBT, but Basher quickly explains that she was going to go to attempt a massacre.
It is also mentioned that Rainbow Basher is obese (however her wing muscles are strong enough to help her fly), although no pictures of her depicted in such away seem to exist yet.
Twilit does not differ greatly from the original character, except for the stripes in her hair (which are yellow instead of purple) and her seeming hatred for the princesses. She is also rather less compassionate than Twilight from the show and marginally more cynical.
Twilit gets her alicornship by sapping the power from Kelestia and Luny (Spoofs of the canon princesses). This renders the two former princesses as averagely sized earth ponies.