Battle for Canterlot

The Battle for Canterlot was an event that transpired in the Era of Harmony when infitrated Canterlot Castle in the guise of  and nearly forcefully married, presumably to gain some dominion over the kingdom of Equestria.

Background
Queen Chrysalis was a magical and unnatural entity birthed from an underground forest over 1,000 years prior to the battle. In those years, she conquered and destroyed many lands, such as Timbucktu and Trot. Equestria was intended to be her next conquest until she failed.

Infiltration of Canterlot
The exact point in which Chrysalis infiltrated Canterlot is unclear, but it was at least some time prior to Twilight and the Mane Six's arrival and after Cadance and Shining Armor decided to get married. Using her power, she managed to put Shining Armor in a sort of trance where he could only follow her orders. Chrysalis hid her nasty, evil personality in plain site by using the excuse that she was merely a stressed bride to waste the time of the ponies who could stop her and sew distrust among them by singaling out Twilight as a paranoid crazy pony. Eventually, she managed to banish Twilight to the Crystal Mines beneath the castle with the real. Twilight was initially distrustful of this Cadance as well, believing her to evil, but quickly realized she was the kind and sweet mare she knew and loved.

This day aria
Eventually Cadance and Twilight managed to get free from their dungeons and outsmarted the Bridesmaids. Although her friends were initially disappointed in Twilight for what they saw as yet another paranoid action, the real Cadance quickly appeared, causing the entire wedding audience to gasp at the reveal. The fake Cadance soon revealed herself to be Queen Chrysalis. Celestia tried to defend her subjects, but lost her uel with the Queen, as the love Shining Armor had for Cadance was far stronger than Chrysalis had believed.

Twilight and the others left the castle to receive the Elements of Harmony and defeat the Queen. Along the way they battled against many Changelings and though they put up a valiant effort, they were eventually surrounded and defeated.

Eventually, Cadance managed to break Shining Armor's trance and together they used their love for one another to repeal the Queen once and for all, saving the land of Equestria from her wickedness.

Aftermath
The battle was recorded by many photographers, with Action Shot taking among the most haunting of photos. Although short, this single battle had a profound effect on the history and culture of Equestria. Chrysalis became a household name and received recognition in Equestria for her evil deeds and would fuel her hate for the land, eventually culminating in her joining the Legion of Doom years later to enact her revenge. Her crushing defeat also caused her to found a new, founded in the ruins of Wuvy Dovey Smoochy Land, which she massacred, and later used new newly found strength to invade Equestria once more.

Behind the scenes
The event essentially took up the entirety of the "A Canterlot Wedding" two-parter and was referenced in several pieces of media afterwards, creating a little storyline in the process dubbed the Changling Wars on the wiki. The event is one of the most iconic aspects of the Friendship is Magic mythos due to its exciting fight scenes, twist and turns, the (then) shocking defeat of Princess Celestia and the sher amount of lore and mythology it added to the series which were only expanded upon later.

Appearances

 * "A Canterlot Wedding"