Elements of Harmony

The Elements of Harmony are a set of incredibly powerful magical stones - comprised of the elements of magic, honesty, laughter, generosity, kindness, and loyalty - that were given by the Tree of Harmony to Crimson Nightmare with the understanding that he was merely one of the guardians of the Elements and not their final wielder. After Crimson disappeared, Princess Celestia became the second bearer of the Elements. She eventually used them to banish her sister, Princess Luna to the moon. Yet she was still not their final owner, and even abandoned the Elements after using them to banish her sister. Their destined owners came in the form of Princess Twilight Sparkle and her friends, who ironically gave them up to the Tree of Harmony, their origin, to save Equestria from Discord's plunderseeds.

Crimson Terror
The one the Elements were bestowed upon, he and the Tree of Harmony had an agreement: That he would be the guardian of the Elements until such time as the next guardian arose, and in return the Elements would lend him power. In the end, Crimson used the Elements to seal Discord in stone, but the stress of one pony using all seven Elements for such a powerful spell was too much, and the spell back-lashed, nearly sealing Crimson too. Fortunately, due to his reasons behind sealing Discord, he became an alicorn prince, which broke the seal, right before being sent back to his home world of Earth, located in the Copy Dimension.

Princess Celestia
After Crimson disappeared, Celestia took it upon herself to guard the extremely powerful Elements of Harmony. She did so for many years, however when her sister turned on her, and she was forced to use the Elements to banish her, she wound up abandoning them in her old castle since they caused her pain in the form of bad memories. Over the years Celestia, the guardian who had abandoned her post, lost her connection to the Elements.

The Mane 6
The true wielders of the Elements, but not their final destination. Twilight Sparkle and her friends were able to wield the Elements to their full potential, even doing what the legendary Crimson Terror had failed to do: Use the stone sealing spell without any negative repercussions. However the Elements were on loan, and the Tree of Harmony called in that loan when Discord's plunderseeds began wreaking havoc.

Below is a list of which element each mare in the Mane 6 wielded.

Twilight Sparkle, the Element of Magic. Applejack, the Element of Honesty. Pinkie Pie, the Element of Laughter. Rarity, the Element of Generosity. Fluttershy, the Element of Kindness. Rainbow Dash, the Element of Loyalty.

Tree of Harmony
The first and final guardian of the Elements, the Tree of Harmony regained it's lent-out power when the Elements were returned to it. Being able to see glimpses of the future, the tree knew that Crimson, Celestia, and the Mane 6 would need the Elements, but also that they would all be able to stand on their own after the Elements were gone.

Power
The Elements are capable of bestowing a significant amount of raw power to their wielders, though this power increases or decreases dependent on the worthiness of the wielder and their reasons behind using them. (i.e. They provided Crimson the power to seal Discord, Celestia the power to banish Luna, ect.)

Spells
While it takes knowledge of how to access them, the Elements are capable of casting spells that their wielder might not be capable of casting. Some of these spells are so powerful that they can seal an Embodiment, though not forever.===

Etching
If the wielder of the Elements is experiencing extreme emotion at the time they're used, they have the curious effect of physically altering the wielder. This has been witnessed in both Crimson Terror and Princess Celestia.

Trivia
In the flashback brought on by Zecora's potion, Twilight saw Princess Celestia and Princess Luna take the Elements from the Tree of Harmony. This was in fact an illusion created by Luna to protect Crimson's identity, and more so her and her sister's falsified images - notably Celestia's, as she knew Celestia would be crushed if Twilight found out and hated her for lying to her.