Artist's description from DA:(Be aware this is strictly the canon for the version of the character appearing in "The Bridge" and its spin-offs or fan works and is not the storyline created by Heilos, her creator)
Eons ago the goddess of creation, Harmony (Left), worked alongside her counterpart Grogar, the god of destruction, to manage the growth and the loss of life upon the archaic world. Harmony would create and foster habitats and environments, whilst Grogar would deconstruct old landforms and sterile biomes into something usable by destroying the old to make way for the new. Eventually life was fostered and blossomed in the increasing complexity until the first sapience developed. Taking it upon themselves to guide and teach the mortal races when they began to take to magic, life was peaceful for several thousand years. The two worked together, but with increasing distance, upon the first three generations. Unfortunately, tragedy struck when the difference in viewpoint came to an event horizon.
But that breaking point hit hard. Grogar, having spent hundreds of thousands of years observing how warble civilizations inevitably felt evil and calamity, often by misuse of his own dark magic; concluded it was not a misuse at all. The creation changed from what it once was meaning it was flawed, and if development caused improvement, then the development of evil was the natural order she had been stopping. The god of destruction became something far worse and launched an assault that exterminated most of the third-generation, a heartbroken Harmony intercepting her counterpart at Ponyville as the survivors fled.
Thousands of years later the descendents of the third-generation, a fourth-generation, unknowingly return to their ancestral homeland to find Harmony and Grogar still in a deadlock, the two perfectly even in power. The six pillars representing different virtues managed to tip the odds and Harmony was able to seal Grogar, the Shadow King, inside the hidden realm of Tambelon. However the battle came at a terrible cost and Grogar's influence was not over. While he had been sealed away his manifests in writings about the darkest type of magic had been distributed to four students. Lust to the Changeling Queen Chrysalis, Greed to a centaur warlock Sendak and later his apprentice Tirek, Gluttony to the chaotic hedonist Discord, and Wrath to the sorcerer Sombra.
Having seen firsthand that mortals were capable of great things and knowing good could use some help against an imbalance of evil, Harmony voluntarily sealed herself away inside a realm* hidden away and only accessible through the Tree of Harmony so she wouldn't be tempted by power over mortals like Grogar had. From there she began to separate away chunks of her power that any mortal with a righteous heart could use.
To the north she created the Crystal Heart to both warm the frozen lands and act as a capstone on top of the entrance to Tambelon. To the heartlands of both the human and equestrian worlds, she separated the white light of her magic into a rainbow of Elements that bear her name: generosity, laughter, kindness, honesty, loyalty, made complete by magic.
Over the years she would separate more and more of her power to better arm and guide mortals, such as the Silver Crystal's blessing of her magic and the creation of a seventh element in the human world based upon empathy. But there were two great gifts she bestowed upon the ponies after the founders returned to their forgotten homeland, two that were extremely dear to her after their creation….
In the modern day Harmony (Right) is still a goddess by every measure. A raw power that can make alicorns and kaiju seem feeble by comparison with only Grogar and the Terran kaiju deity of Bagan as comparable. However compared to her old self, she has been drastically reduced and she shows no signs of stopping. There was always a temptation to take back her full power, take back all of her gifts, and solve the problem like an ant under a boot. But, she resists. After all if beings as short-lived and relatively feeble as mortals could do such amazing things in her eyes, the world is in good hands and hooves. She'll just keep making the right tools to fit them.
Despite the new challenges, with creatures from worlds as alien to each other as they are to the equestrians, she does not despair. So long as there's capability within a heart to beat, pure evil can never overpower the light. And so long as the light forged by bonds of friendship or love can exist, there always will be a place for Harmony.
*It's the same place Celestia ascended Twilight, a starry void within the Tree