The Goddess AU is an AU I had an idea for where Pinkie Pie is the spirit of chaos instead of discord and Princess Celestia, growing desperate to free her sister from the moon and her curse, enlists Pinkie’s help to find the Elements of Harmony’s new bearers. Together they find they young mane six and teach them to wield the elements to save Luna.  
 
There are multiple sources of power in the world, but the most powerful of all are Goddesses and Spirits, the forces that control Equestria. There are also Immortals, mortal ponies(or other creatures) that have ascended, gaining power near to that of a Goddess or Spirit.  
Both Goddesses and Immortals cannot die of natural causes, but can be killed. Immortals will stay dead, while Goddesses and Spirits will reincarnate or be replaced. Goddesses and Spirits were never mortals - the universe created them. They age incredibly slowly and can move through time fluidly, can take on different forms, and have immense magical power.  
 
Celestia and Luna are the most well-known Immortals. Sister Immortals, they ascended together and rule the day and night respectively. They have been around for about two thousand years. A thousand years ago, Luna became corrupted by a terrible curse that turned her into Nightmare Moon. Celestia had to seal her in the moon, but now, a thousand years later, she desperately wants to be reunited with her sister, the only constant in her long life. There is a third princess - Princess Cadance, who rules the Crystal Empire and is the princess of Love. Princess Cadance, or Mi Amore Cadenza, as her name really is, is not an Immortal. Well, everyone THINKS she is, she looks like an alicorn like most Immortals, and she’s a princess like the sisters. In actuality, Cadenza is the Goddess of love and connection between ponies, and has been around since civilization has, really. She’s only recently come into the public eye and everyone has forgotten that she’s not actually an Immortal.  
 
The Everfree Forest is one of the only places ponykind do not control with their innate magic. Pegasi don’t control the weather, Earth Ponies don’t spread their fertile plant-growing magic through their hooves, and yet it still functions, all on its own. Incredibly dangerous and barely explored, the tangled monster-filled jungle is home to several spirits. Zecora, the Goddess of the Wild, who is the only one the Everfree will listen to, and Ahuizotl, the Spirit of Beasts, who created many monsters.  
 
Mistmane is another very old Goddess. She is fading out, as the Goddess of Generosity has been replaced with the Element of Generosity. She was the one who raised and tutored the new and young Goddess of Chaos, Pinkie.  
 
Discord, the old spirit of chaos, was stripped of his power by Starswirl the Bearded and banished to another dimension, where he now stews bitterly over being replaced. Chaos magic is a neutral force in Equestria, one that runs many monsters and plants and flows throughout it. It is neither good nor bad, simply existing, bringing spice to life. However, Discord used it to terrorize and destroy, and it took a long time before he was able to be trapped and his magic taken.  
 
Stygian was another Immortal that was stripped of magic. The Pony of Shadows, a long-lasting legend, the scary creature that kept ponies safe and inside, Stygian was the Immortal of Death. Like a grim reaper, Stygian’s job was to collect lost souls and keep little ones inside when the sun went down. Starswirl says that Stygian grew violent and spread fear to all the ponies, and so he took Stygian’s magic, transforming him into a normal pony once more, imprisoned in Tartarus.  
 
Adagio, Sonata, and Aria were a trio of sirens that created conflict and wars to feed on the negative energy. They were not Goddesses or Immortals, simply powerful, long-lived monsters. However, Starswirl advanced on them to fight, sending them to a realm without magic where they cannot hurt anybody. Without their powers, they are frightened of the new world. Adagio, the leader, tries to keep her lovers safe in this strange dimension.  
 
The great Starswirl himself, Spirit of Magic. Or… at least that’s what everybody thinks. Starswirl was once a mortal. He hides his alicorn wings, a telltale sign of Immortals, under his cloak, or casts illusions on himself. He hid all traces of his past life, swore his brother Stygian to secrecy, and, when Stygian grew doubtful, Starswirl took his magic and locked him in Tartarus. Starswirl is a Spirit, the strongest Spirit, the Spirit of Magic, and he holds the power to give and take magic from the others as he so wishes. If the other Goddesses and Spirits found out that he was just an Immortal, posing as the leader, they would surely ruin him, turn him back into a normal unicorn.  
When Tirek, Spirit of War, and Scorpan, Spirit of Justice, came to Equestria to steal its magic, Scorpan saw the beauty of the land and went behind his brother’s back to tell Starswirl of Tirek’s plot. Tirek was imprisoned in Tartarus for his crimes, and Scorpan was allowed to go back home… Is how history tells the legend. Once again, Starswirl has rewritten history to suit his lies. Starswirl was jealous and afraid of the gargoyle’s power, and so kept him close as an ally rather than allow him to go free among the ponies. Scorpan sensed Starswirl’s lies and tried to tell the others about this fraud in their midst, but Starswirl threatened Scorpan. Starswirl, the greatest sorcerer and alchemist of all time, could easily suck Scorpan’s magic out. Disgusted, Scorpan flew away from Equestria in the night, taking Tirek with him, and the two returned to the Centaur Lands. Starswirl declared Equestria and the Centaur Lands enemies, telling the other Spirits they had declared war, and together they constructed a magical barrier to divide Equestria from the Centaur Lands to keep Scorpan and Tirek from returning. This has mostly been forgotten by the others, and now the shield is weakening.