Chest of Harmony

The Tree of Harmony chest is first seen in Princess Twilight Sparkle - Part 2 after emerging from the Tree of Harmony. The chest requires six keys to open.

Princess Twilight Sparkle - Part 2
When the Tree of Harmony regains its magic, a flower sprouts at its roots. The flower blooms to reveal a chest with six locks. Princess Celestia discloses that she does not know where the keys are but assures Twilight Sparkle "...it is a mystery you will not be solving on your own".

Castle Mane-ia
The chest is briefly mentioned in the beginning of the episode. According to Twilight, no information on the chest and its contents exists in Ponyville.

Twilight Time
A blueprint of the chest hangs on a wall in the Golden Oak Library.

Maud Pie
The same blueprints as in Twilight Time are seen again in Maud Pie.

Twilight's Kingdom - Part 1
With the looming threat of Lord Tirek, and at the prompting of Discord, Twilight Sparkle redoubles her efforts to find the keys to the Chest. However, she is unable to find any clue in the library of the Castle of the Royal Pony Sisters. Once again following a clue from Discord, Twilight decides to check the journal in which she and her friends are recording their lessons in the magic of friendship. Discord has bookmarked five events that included fundamental tests of her friends Element virtues and led them to teach these virtues to other ponies.

The Mane Six bring the five gifts they received as a result of these adventures. When Pinkie Pie accidentally knocks Boneless against the mysterious Chest, the toy transforms into a key. All the other gifts do the same.

Twilight's Kingdom - Part 2
With Tirek threatening her friends if she does not surrender, Twilight Sparkle chooses to give up her magic rather than see them harmed. Much to Tirek's surprise, she includes Discord in her bargain despite his having betrayed them all. A chastened Discord gives Twilight a medallion that Tirek had given him as a false sign of friendship, telling Twilight that, for him, it truly is a symbol of true friendship.

Seeing the rainbow shimmer on the medallion, the friends realize that this is the last of the keys. They rush to the Tree of Harmony where it transforms into the sixth key. The friends turn their keys simultaneously and the chest opens, releasing a rainbow-like blast of power that transforms them, giving them new magic and rainbow-marked coats, manes and tails.

Tirek arrives but even with the power of every pony in Equestria behind him, he is unable to penetrate the combined shield of the six empowered friends. A blast of rainbow light strips him of his stolen power and returns him to Tartarus. The friends then unleash a rainboom-like wave of power that restores the magic to all the ponies of Equestria.

After Tirek's defeat, a large rainbow from the Tree of Harmony carries the chest and its keys to Ponyville, where it sinks into the ground and a grand castle rises from where it came to rest.

Keys
Following the time of the chest's discovery, Twilight and her friends come into possession of several objects of assorted value. The ponies acquire these objects after facing situations that challenge them to live up to their respective Elements of Harmony and, upon embracing their element, inspire other ponies to do the same. Prior to the acquisition of these items, each pony has a moment of clarity relating to her element that manifests as a rainbow-colored glow in her eyes, and the items themselves emit the same glow soon after the ponies receive them.

Rarity – generosity
In Rarity Takes Manehattan, Rarity wins Manehattan's Fashion Week, despite the schemes of fellow designer Suri Polomare and her own poor treatment of her friends as a result. Designer Coco Pommel is so inspired by the generosity shared between Rarity and her friends that she quits her job as Suri's assistant and seeks out a better life in the big city. As a sign of her gratitude, Coco gives Rarity a spool of rainbow-colored thread.

Rainbow Dash – loyalty
In Rainbow Falls, Rainbow Dash confronts Spitfire and Fleetfoot of the Wonderbolts over their unethical means of recruiting her to Cloudsdale's aerial relay team, and chooses Ponyville over them. Inspired by Dash's loyalty, Spitfire and Fleetfoot apologize to their teammate Soarin for trying to replace him. As a token of her appreciation, Spitfire gives Rainbow Dash a golden Wonderbolts pin.

Pinkie Pie – laughter
In Pinkie Pride, Pinkie Pie feels challenged by fellow party planner Cheese Sandwich and allows her pride to get in the way of planning Rainbow Dash's birthday party. When Cheese Sandwich makes it clear that he never wanted to take Pinkie's place in Ponyville, and tells how one of her parties motivated him to become a party planner as a colt, the two make amends. He later gives her his rubber chicken companion Boneless as a memento of his visit.



Fluttershy – kindness
In It Ain't Easy Being Breezies, Fluttershy evicts a swarm of Breezies she'd been caring for after realizing she is jeopardizing their home migration with her pampering. In the meantime, she teaches the loudmouthed and rude Seabreeze to be kinder to his fellow Breezies, and to only be firm with others when all else fails. After Fluttershy helps the Breezies return home, Seabreeze gives her a flower with blue petals to remember them by.



Applejack – honesty
In Leap of Faith, Applejack becomes entangled in an attempt by Flim and Flam to sell a worthless vegetable tonic as a "cure all" medicine. When this unwittingly leads to Granny Smith behaving rashly under the tonic's placebo effect, Applejack confesses to having misled the ponies of Ponyville. In response to her honesty, Silver Shill, the brothers' accomplice, backs up her denunciation of the con artists. He then gives her the bit he took from a pony for a bottle of the tonic, and promises to repay his customer from his own funds.



Twilight Sparkle – friendship
In Twilight's Kingdom - Part 1, Twilight Sparkle's attempt to find her key is stymied by the fact that she had never faced a situation testing her magic. In, Twilight's Kingdom - Part 2, Lord Tirek forces Twilight to bargain for her friends' safety in exchange for her alicorn magic. Twilight accepts and includes Discord in this bargain, despite the fact that he had betrayed them. Penitent and broken, Discord gives Twilight a medallion that originally belonged to Tirek's brother Scorpan and that Tirek had given him in false friendship.