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Zombietale is an AU created by Zombie Frisk. The only change there is within this AU is Frisk is a zombie. However, there is no information on how they become a zombie, but theories include that the earth is now a post-apocalyptic timeline where zombies rule supreme or that Frisk had died in the fall and returned as a zombie, the only evidence there is would be a bullet hole in Frisk's forehead.
Frisk falls down after being shot it the head, and is discovered by Toriel. She tries to heal Frisk, only to cause more pain for them, their soul beyond repair and far too broken. When she realizes there is nothing she can do, she spends time with Frisk. reading them stories and baking them pies. They aren't interested though, they only focus on their hunger as it begins to grow monstrously.
When Toriel falls asleep, they leave the house and feed on a Froggit, transforming it into a zombie. The Zombie Froggit goes on a rampage, biting other monsters and spreading the infection. A Whimsun named Sunny escaped the destruction with the help of Flowey and she was taken under his wing.
Toriel eventually finds the infected monsters in the ruins. She runs away and locks herself and Frisk in her home, warning Frisk about the undead monsters. When she realized that Frisk was actually the cause of the infection, she fought Frisk in order to protect the remaining monsters.
Frisk spared her and Toriel took them into a hug, promising that they would fix everything. Frisks hunger takes over though, and they bite Toriel when she least expects it, leaving them feeling hungry and empty.
Frisk is the main character of the AU, and is the only one that is changed from the original Undertale storyline. They are a zombie, and can't die. Healing magic does not work on them and only causes them pain. While seeming to only care about their hunger, they have been shown to feel regret about their actions, namely when they bite Toriel.
Flowey is relatively unchanged from Canon, but he plays another role in Zombietale. He is somewhat of a hero, saving Sunny from the zombie hoard and figuring out ways to actually kill the zombies.
That was, before his untimely zombification after coming in close contact with Frisk, who he mistook as Chara.
Sunny is a Whimsun that managed to survive the zombie apocalypse. Her sister, Luna, was killed by the Zombie Froggit. She was saved by Flowey, who stumbled onto the scene and carried her under the underground where it was safe.