Evil Celestia is the 2nd episode of the 3rd season (1st being my Transformers crossover) and is the first episode to revolve around Princess Celestia. It's also the second episode where one of the major characters turns evil (1st being Spike in Secret of My Excess)
A heat wave has been making Equestria very hot, and all the ponies are trying to find ways to keep cool during the hot hours. This is because of the fact that this is the time of year when the sun is closer to Equestria than ususal. Twilight Sparkle is sending her usual letter to Princess Celestia, but she does not get a response. Twilight suspects that Celestia must’ve been kidnapped, so she gets all the other Mane 6, Rainbow Dash, Applejack, Fluttershy, Pinkie Pie, and Rarity, to go to the castle with her and investigate. They ask a guard where Princess Celestia is, but the guard tells her she’s been acting very strange lately. She has been very tempermental and angry, and her temperature is rising at an alarming rate. The ponies go into the castle and spot Princess Celestia, who is acting weird. It’s too hot to be around her because she is giving off enormous amounts of heat to everything around her. Not even Celestia knows what’s happening to her, so Princess Luna and the other ponies take her to see Zecora.
Zecora reveals that Princess Celestia’s powers revolve around the sun, due in part to her responsibility of raising the sun every dawn of day. Because of the heat wave coming from the sun, Princess Celestia is absorbing the sun’s rays, making her become more and more powerful than usual. The sun’s super-hot rays are also making her evil. The heat wave temperature eventually increases, causing Princess Celestia to transform. Her tail and mane are now made of fire, and her eyes turn red. She also becomes really evil. Princess Celestia breaks through the roof, and she gains fire powers. Princess Celestia starts burning down buildings in Ponyville. Princess Luna tells the ponies that only the Elements of Harmony can return Princess Celestia to normal. Princess Luna quickly tells the ponies to get the Elements of Harmony and she will keep Princess Celestia busy. Princess Celestia continues burning Ponyville to the ground, and Princess Luna confronts Princess Celestia, who engages Luna in combat. Princess Celestia overpowers Princess Luna, and easily defeats her. Thankfully, the ponies arrive with the Elements of Harmony. Princess Celestia attacks the ponies, but they quickly use the Elements of Harmony on Princess Celestia, returning her to normal. Princess Celestia sincerely apologizes for the damage and destruction she’s caused, and promises to help rebuild. Also, the heat wave finally ends.
So, what do you think?