How the demon arrives and how is trapped, as well as the origin of the book, killing starlight in the process
Starlight jealious of her freind for having learned magic first, goes to a journey to gather as many magic abilities as possible, so she founds about an ancient artifact that can steal the magic from the cutie mark of ponies, she embarks a searsh to find it, on her way, she starts gathering books of spells, meanwhile a demon arrives to ponyville in form of a meteorite, as the demon still being week, it wa easy for zecora and lyra to trap it into a bottle with help of an old hidden book buried deep in everfree forest, the "Worldirium", but the book is corrupted in the proces. Lyra brings back the bottle with the demon home and buries down the book again in everfree forest, unfortunetly, she does a porly job and the book is soon founded by a merchant, and it goes hand to hand until it arrives to stalight glimmer, who begins to read it, but she became more and more corrupted, at the point of death because the demon consumes the ponies who use the book, so the demon can survive being trapped into the bottle, several years after, the book was passed down hand to hand, until it arrived to chanterlot, where Twilight finds it, but before being able to read it, celestia mandates twilight to go to ponyville and make some friends.
How the demon is unleashed and how it posees scotaloo, killing rarity in the process
Twilight arrives Ponyville, as well as nightmare moon, unraveling the demon trapped in the bottle, the demon still being vey weak, so it starts consuming the souls of everypony on ponyville one at the time and silently, just like the first time, but wiht even more caution, as it dont want to be discovered by Zecora or Lyra, meanwhile when the sonic rainboom debacle happened to rarity (the spell that will allow a pony to fly for 3 days to cheer rainbow dash up on cloudsdale) (MLP:S1-EP16) Rarity borrows more books from Twilight, from fashoin design, to some spell books out of curiosity, one of those was the "Worldirium". After a while of the demon being undetected (aprox a couple of weeks), it decides to take a risk and try to cosume a pony: Scotaloo (wich was choosed randomly) so the demon tries to consume her to death, but it fails, since it stills weak, so the demon tries something diffrent, to make a deal, so the demon presents itself to scootaloo, the demon offers her a cutie mark in exchange of her soul, but she wants her family back instead, the demon agrees, so scootaloo shall become the new puppet of the demon. scotaloo the next day goes to a sleepover for the Cutie Mark Crusaders, it was going to be in rarity's house, but fluttershy offers to watch over the crusaders on her house, because she was very busy on one of her jobs, also she seemed more weak than habitual, with bags under her eyes and not much motivation, because she has been reading the Worldirioum, the demon then discovers she was the new owner of the book thanks to scootaloo, so sets its new victim to be rarity, in order to consume her and gather the book for future victims, so scootaloo agrees and the following day after the cockatrice debacle (MLP:S1-EP17) she decides to go back rarity's house to get the candies bag she "forgot" on Rarity's house but at the end of that night she ends with rarity's life, now with a freshly dead body is way easier for the demon to extract the remaining vitality of the body, gaining more power in the process, but when it came the moment to search the book, both the demon and scootaloo didnt found much sucess because it was turning sunshine and scootaloo needed to go to school with the other ponies, or they will find her with the dead body and would enprison her, and maybe find and capture the demon again, so she decided to make rarity's death look more like a suicide, but it wasnt good enough, leaving some items behind [one of the candies that accidentally fell off, the pills she used to bring rarity to sleep before killing her and X]
How the investigation begins, as well as applebloom kidnap and scotaloo breaking the team apart
The body of rarity is descovered without life, so a funeral is held, everypony in poniville is depressed, during the funeral services and rituals, twilight discover the clues in the house of rarity that lead to the conclussion that it wasnt a suicide, so decides do start digging more, Meanwhile, the demon gains vitality with the death of rarity, regardless of scotaloo not being able to find the book in time, but because it has more power, is able to posses and to disguise itself inside Scootaloo, Meanwhile a depressed sweetie belle gets home after the funeral of her sister but is aproached by a pissed off Rainbow Dash who told her to go to Zecora's hut, she accepts, Rainbow dash is talking to herself and crying the whole trip, once they get inside Zecora's hut, the athmosphere turns dark and dense as Rainbow dash ans Zecora Turn against her. The cursed book has consumed Zecora, the contact she had with the book just after getting cursed made her lose her mind slowly and painfully, making her think that the only way to get her sanity back is by a ritual to sacrifice a young female unicorn, so when she walked thowards ponyville in search for one, she finds rainbow dash passing by just after the funeral, and knowing that some ponies still scared of her, she decides to told rainbow dash a lie, that she would bring Rarity back but with a price. Sweetie belle finds the danger soon and escapes before the ritual even began, so she runned away to rarity's tomb to find real comfort, but a thought strart to invade her mind, the book, she remembered that since rarity has been reading it, she started to look sicker and act less motivated, so she wanted to find it and burn it, so she runs back home in search for the book, meanwhile the demon in scootaloo's body makes a visit at Applejack's home with the excuse to talk and find comfort with Applebloom, as she is her "friend" but as soon both crusaders were alone, and applobloom started crying by the loss, the real goal unfolds, the demon wanted to get her and get more power, but right before she could end with applebloom via scootaloo, Applejack interrupts by going inside the room to bring them snacks and to talk with the two young ponies as well, but she also starts crying, then the demon realizes that maybe it can end 2 birds with one shoot, so instead of aiming only applebloom, the demon targets applejack as well, so the demon decides to talk to scotaloo, scootaloo tells the demon that should get applejack to search for the book and that
gg
that's everything i got so far :p