HOW THE EPISODE WOULD GO:
 
Diamond Tiara finds the Alicorn Amulet being used as Zecora’s new doorstop, takes it, and gains a unicorn horn and magic, becoming even worse than usual in the process. she quickly becomes an equestria-level threat and strikes down Celestia and Luna. she also revives Nightmare Moon and King Sombra, enlisting them and Queen Chrysalis as her cohorts with a mind-control spell. Discord quickly alerts Twilight and the mane 6.
 
Much later, in the 2nd part, Diamond Tiara decides to sap the power of Discord after being beaten in combat by Twilight. in doing so, she also assumes the powers of her mind-controlled slaves trying to catch him. eventually, Diamond Tiara succeeds and steals all of Discord’s power, merging herself with the Alicorn Amulet and turning her into the monstrous Tiara Ultima…
 
Of course, this is just an idea…