When King Sombra was defeated some of the magic of the Crystal Heart entered his heart, opening it. When King Sombra returned he was filled with remorse and chose to live on an uninhabited island far away from other ponies because he feared that nopony would forgive him. With his magic he turned a mountain in his own castle and four stone statues he’s made into servants and knights. Celestia, with a new telescope given to her by Discord, found King Sombra’s Island and Discord went to check it out. Discord returned to the Royal Sisters and told them how bad King Sombra feels. They were surprised at first, but they believed that he is really turning good. Everyone soon heard of King Sombra’s return, even the Mane Six and after a few weeks on the island King Sombra decides to disguise himself and go to Ponyville. Princess Luna talks him out of disguising himself and gives him a medallion so he can call her anytime if he needs her. King Sombra goes to the Friendship Rainbow Kingdom and started spending time with the Mane Six. Since then he started doing good things: he helped a colt that was being bullied, helped with the harvest at Sweet Apple Acres and had brunch with the Apple Family, played the piano at the spa, helped Rainbow Dash with a stunt and saved her from crashing into Ponyville, played the pipe organ for the Mane Six and giving it to them, and then word started going around that King Sombra has reformed. Word even got to the Crystal Empire, but Shining Armor couldn’t forgive him after what he had done and for what he put Cadance through. When King Sombra was ready he went to the Crystal Empire and apologized for what he had done and was willing to be exiled, but Cadance forgave him and told him what the Crystal Heart did to him. Shining Armor shook hooves with King Sombra saying if Cadance forgives him he will too. King Sombra gave Cadance a silver bracelet with a ruby so Cadance can call him if she needs him and told them that they have made allies in the dark. King Sombra felt so good that he threw a ball at his castle, inviting almost everyone in Equestria and danced with Luna. Soon he had a vision about something terrible happening to the Crystal Empire and went to check it out. The Crystal Empire was fine and he found a dragon egg under the thrones. He only remembered putting it there before his defeat but he couldn’t remember the reason why and was put in charge of the egg. King Sombra did doing more good things afterwards: bonding with Apple Bloom when she had the Pony Pox, fighting Captain Blackmane to save Rarity, help repair Ponyville after Frankencorn attacked, helped Cadance get to the hospital when she was ready to have her baby, he proposed to Princess Luna, help defeat Nightmare Moon II, help reform Starlight Glimmer, help set free Princess Isle, becoming leader of Our Town, defeating Starlight Glimmer’s grandmother, and so on. When the Dragon Egg hatched he remembered why he put it in the Crystal Empire and where the egg came from. With a piece of his mane and dark magic King Sombra created a perfect Dragon Clone of himself before he was defeated and somehow put in the Crystal Empire. He warned the Crystal Empire of the Sombragon coming and fought the Sombragon himself in his Shadow Form. The Sombragon got the upper claw and took down King Sombra. Even though he was hurt he stood up ready to fight. The Sombragon breathed fire at Sombra, but Luna took the hit and was knocked unconscious. When she wouldn’t wake up King Sombra went into an uncontrollable rage and let the darkness into his heart again. As a result he turned into a beast, but he remembered who he was fighting for. He fought the Sombragon exhausting each other in the process. King Sombra held the Sombragon still and ordered the Mane Six to use their Elements of Harmony. They reluctantly did and when the Sombragon was turning to stone King Sombra jumped out of the way. When the Sombragon turned to stone completely King Sombra, in cold blood, reduced him to rubble. He laughed on last evil laugh and collapsed. King Sombra turned to his normal self, but the darkness was to much for him and he was slowly turning to stone. Luna regained consciousness and King Sombra said his goodbyes to her and those he grew to care and love. After saying he’ll always love Luna and the others he turned to stone completely and fell apart. Luna screamed in anguish and begged King Sombra not to leave her and said she loves him. Everyone in the Crystal Empire shedded tears, even those who have not forgiven King Sombra until they saw how he sacrificed himself. The Crystal Heart made those tears enter King Sombra’s stone remains and recreated King Sombra. King Sombra was reborn a Crystal Pony. He and Luna kissed and the damage the Sombragon caused was corrected magically. The Sombragon was also revived as a colt looking like King Sombra before he became a Crystal Pony. King Sombra and Princess Luna took him in and named him Prince Umbra and they got married.
 
The story is The Redemption of King Sombra