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The story of the dark unicorn King Sombra from My Little Pony: Friendship is Magic.
King Sombra's early life is left unknown in the TV series, unlike the comics. It can be speculated that Sombra was once a resident of the Crystal Empire but was corrupted by dark magic and it led to him becoming an evil power-hungry Unicorn. It is also implied that Sombra murdered the previous Queen of the Crystal Empire so he can take over the Empire.
King Sombra was a powerful unicorn whose heart is said to be as black as night. He took control of the Crystal Empire and enslaved the crystal ponies before Celestia and Luna overthrew him, turning him to shadow and banishing him to the Arctic North, disappearing into a crevice of ice which closed up. However, he foresaw this defeat and placed a curse upon the Crystal Empire that caused it to disappear for a thousand years.
The Crystal Empire
In "The Crystal Empire", the Crystal Empire reappears, but the crystal ponies have no memory of their kingdom before Sombra took over, and the mere memory of him causes them pain. Sombra returns along with the Crystal Empire and tries to conquer it again, but Princess Cadance was able to cast a protection spell around it which kept him out. When Twilight Sparkle and her friends came to the Crystal Empire to protect it, Sombra chased after them in shadow form, but they got through the barrier, although he attacked Shining Armor he survived, but his horn was cursed so that he could not use his magic.
When Cadance began to run out of power to perform the spell, the barrier briefly deactivated, allowing Sombra to invade. Sombra's horn was broken off as the barrier reactivated, and he transformed it into a dark crystal which began to create more dark crystals in the Crystal Empire.
Twilight and Spike searched for the Crystal Heart, the key to protecting the empire, inside the castle, where nopony would dare to look. They ran into a door enchanted by Sombra to hypnotize them into confronting their worst fears. In Twilight's case, she thought Celestia told her that she failed her test and wouldn't continue her studies, only for Spike to snap her out of the trance. Spike gets brainwashed too and thought Twilight was banishing him, but Twilight saw through the enchanted door's purpose and shuts it, releasing Spike from the spell and comforting him about his fear.
Eventually, they reached the Crystal Heart, but Sombra, was finally able to enter once Cadance was no longer able to perform her spell, trapping Twilight in a wall of dark crystals, keeping her from the Crystal Heart once she triggers a trap.
With no way out, Twilight entrusts Spike to take the Crystal Heart to Cadance. Returning to his physical form, Sombra tried to spear Spike with a dark crystal, but Cadance saved him from the attack. She then returned the Crystal Heart, and together with the crystal ponies used its power to unleash a wave of magic that restored the Crystal Empire and destroyed Sombra, shattering him to pieces.
King Sombra was briefly referenced in the episode, "Games Ponies Play". Twilight also briefly uses his magic in "Princess Twilight Sparkle".
The Cutie Re-Mark
In "The Cutie Re-Mark", the first of many alternate timelines temporarily created by Starlight Glimmer, King Sombra is in power and ruling over the Crystal Empire with the iron hoof, having never been killed by Princess Cadance due to the change in timeline, he made the crystal ponies fight for him against the rest of Equestria, fighting a war with Celestia. He was winning the war as he managed to take half of Equestria.
King Sombra was mentioned again couple of times in the Season 6 premiere "The Crystalling". Spike mentions King Sombra while telling the Crystal Ponies about how he was defeated. Twilight later reveals the reason that King Sombra merely hid the Crystal Heart in his castle is to prevent the Frozen Norths weather from breaching the Empire. This explains why Sombra never destroyed the Crystal Heart in the first place.
In the season finale, "Shadow Play", Pinkie Pie mentions Sombra while recounting the threats Equestria has faced over the last thousand years to Starswirl the Bearded.
The Beginning of the End
In "The Beginning of the End", King Sombra is revived by Discord (who was in the form of the ancient tyrant Grogar) and is summoned to his lair, where Queen Chrysalis, Lord Tirek, and Cozy Glow accuse him of being the one who summoned them there. However, "Grogar" reveals himself as being the one to summon them, much to the villains' astonishment, as they had all thought he was a legend (with the exception of Cozy Glow, who had never heard of him before). "Grogar" proceeds to tell the group of his plans to eliminate Twilight Sparkle and her friends and demonstrates his power by giving Tirek enough magic to partially revive his strength, also causing him to tell Cozy Glow of the stories that he had heard of "Grogar" when he was younger and for Sombra to do the same afterwards. "Grogar" goes on to state that he's been watching over all of them and asks them why they think that they've always been beaten by the same six ponies, Chrysalis interrupting Sombra before he can respond. "Grogar" rebuffs all of their answers and points out that the Mane 6 have always won because they work as a team, and demands that they all do the same in order to take over Equestria.
However, Sombra refuses to join "Grogar", claiming that he will take the Crystal Empire back for himself and destroy anyone who gets in his way. Unphased by this, "Grogar" tells Sombra to go and try to take over the Empire again, even offering to transport him there on the conditions that he can keep it if he succeeds, but will serve him once he fails, claiming he will destroy him again if he refuses to abide by his terms. Sombra agrees and proceeds to arrogantly proclaim how he will overtake the Empire, before an annoyed "Grogar" whisks him away. Sombra proceeds to lay siege to the Crystal Empire with ease, brainwashing the crystal ponies and royal guards into serving him as he approaches the castle. Inside, Princess Cadence sends out a message to Twilight before she and Shining Armor notice that Flurry Heart is missing and find her in Sombra's clutches. Sombra then forces Shining Armor and Cadence to surrender, capturing the Crystal Heart and fully taking over the Crystal Empire afterwards.
Upon receiving Cadence's message and summoning her friends, Twilight decides to retrieve the Elements of Harmony from the Tree of Harmony to defeat Sombra. Back at the Empire, Sombra is seen relaxing on his throne, amused by Flurry Heart's attempts at attacking him, only for the Mane 6 to arrive to challenge him. Unconcerned, Sombra forces his opponents to endure their own worst fears (making Twilight see a vision of all of her friends being controlled by Sombra while Princess Celestia and Luna show disappointment in her, turning them into Nightmare Moon and Daybreaker as a result), and suggests that they should surrender, as nothing can beat their own fears. However, Twilight and the others overpower Sombra's visions with the Elements, revealing that they were only facing their fears long enough for Spike to free his captives.
Cadence then reclaims the Crystal Heart and uses it to dispel Sombra's influence from the Empire, freeing everyone from his control. In spite of this, Sombra proclaims that he will win in the end, before being seemingly destroyed by the Elements. Having saved the Empire again, Twilight and the Mane 6 return the Elements to the tree, only for Sombra to appear and destroy both.
Sombra reveals that his supposed demise back at the Crystal Empire was merely a ruse to fool the Mane 6 in so that he could trace them back to their source of power and destroy it; and now that he has done so, they will be unable to stop him. Despite this, the Mane 6, minus a disheartened Twilight, try to fight back against him, only for Sombra to imprison them all, realizing that, with them out of the way, he can conquer all of Equestria for himself, planning to overtake Ponyville first. Sombra then considers using his magic to brainwash the Mane 6 as well, but decides against it, as he is already forcing them to endure their worst fear by taking control of Equestria anyway before leaving. Sombra goes on to brainwash all of Ponyville — including Spike, Starlight Glimmer, the Cutie Mark Crusaders and the Mane 6's other friends—into becoming his slaves and takes them to conquer Canterlot, while the Mane 6 are forced to deal with the Everfree Forest's overgrowth thanks to the Tree's destruction.
Sombra storms Canterlot with ease thanks to his brainwashed army, and barges into the Princess' throne room, only to find that it is empty, making him believe that Celestia and Luna had run away from him in fear. Soon, the Mane 6 arrive in the Princess' throne room with Discord (back in his true form) and engages Sombra, who attempts to blast them with magic. However, Discord dispels all of Sombra's attacks with ease while commenting that, even though he expected Twilight and the others to handle things by themselves, they still win because he is their friend, though in doing so he accidentally prompts Sombra to target Fluttershy instead, making Discord take the blast for her. Sombra gloats at his victory and begins to decide how he will eliminate the rest of his enemies, while, at the same time, Discord gives his friends a heartfelt speech about how they don't need him or the Elements to beat Sombra since all they have ever needed to win is each other.
Disregarding Discord's claims, Sombra attacks the Mane 6, who, now inspired, begin to fight back, generating more magic than ever. Baffled, Sombra demands to know how the Mane 6 have the ability to stand up to him even when he destroyed the source of their power, although Twilight claims that he never did, with her and the others reminding him that friendship is magic, before using their power to destroy him once again and remove his influence from Equestria. Due to how powerful the Mane 6's magic was, Sombra's skin was tearing itself apart while he quickly turns to dust as well and disappears into the light, destroying him once and for all.
Later, Celestia and Luna return from fending off the Everfree Forest and state that they will hold off on making Twilight and her friends the new rulers of Equestria until Twilight feels she is ready, causing Discord to reveal that he faked his injuries in an effort to push Twilight to become the great leader he knows she is, additionally revealing that he could have defeated Sombra any time he wanted. Back at "Grogar's" lair, Tirek and Chrysalis ask him about how Sombra's plan went, and "Grogar" shows them that he had been defeated yet again. Choosing not to revive him a second time, Discord (back in his Grogar disguise) uses Sombra's destruction as an example to the others of what happens when they try to abandon his plans, making them nervously agree to serve him.
King Sombra is mentioned a few times in the episode, "Uprooted".
He is also mentioned in "Sparkle's Seven", where it is shown that Princesses Celestia and Luna decided to have Shining Armor increase security in Canterlot following Sombra's previous attack on it.
In "She's All Yak", Sandbar lists King Sombra and Cozy Glow as two ponies worse than Yona.
He is briefly mentioned in "The Last Crusade" by Mane Allgood, who lists him as one of the main reasons why she and her husband Snap Shutter decided to come to Ponyville and spend time with their daughter, Scootaloo.
In "The Ending of the End - Part 1", Sombra is mentioned a number of times when it is revealed that Discord had been posing as Grogar and was using him and the other villains to create a threat that would push Twilight and her friends into becoming confident rulers of Equestria. In "Part 2", Twilight has a picture of Sombra included in a thought chart when trying to come up with a way to defeat Tirek, Queen Chrysalis and Cozy Glow.
Depiction in the comics
King Sombra of a parallel world appears in the seventh arc (Issues 17-20) of the comics. He is the polar opposite of the Sombra from the show, as he is a good ruler of the parallel world where Celestia and Luna are evil. He is in love with Celestia of Prime Equestria.
However, this version of Sombra would suffer a terrible fate in allowing himself to be taken by evil to protect his beloved Celestia and both their worlds from destruction, becoming just like the King Sombra from Celestia's world. But even after being taken by evil, this Sombra mourns his beloved Celestia, now lost to him, possibly forever.
In Friends Forever #4, it is shown how King Sombra captured a Crystal Bard and enslaved him, making him read books for him, after the Crystal Bard escaped King Sombra with his royal guards trying to recapture him but the Crystal Bard became smarter than him.
Fiendship is Magic
In Issue 1, Sombra's backstory (at least in comic canon, in which Amore has a different cutie mark than described in the Journal of the Two Sisters) is revealed; he was an Umbrum disguised as a pony to enter Equestria and release the other Umbrum, who were imprisoned in another dimension. Sombra initially didn't want to turn evil, and was friends with Radiant Hope, and was affected by the power of the Crystal Heart, but after a talk with the Umbrum, he embraced his dark side. He turned Princess Amore to crystal and shattered her to pieces, before being banished by Celestia and Luna, but he threw Radiant Hope outside of the Crystal Empire. In the present, it is shown that Sombra is now a ghost tied to his horn.
Siege of the Crystal Empire
In Issue #34, Radiant Hope recruits Queen Chrysalis, Lightning Dust, Iron Will, and the Flim Flam Brothers to attack the Crystal Empire as a distraction while she finds Sombra's spirit and horn, and uses her healing magic to bring him back to life.
In Issue #35, Sombra releases the imprisoned villains, and he encrusts Twilight's horn with dark crystals, allowing Chrysalis to defeat her. Celestia and Luna arrive to fight Sombra and Chrysalis, but due to Hope's intervention, Sombra turns Celestia and Luna to stone.
In Issue #36, Sombra and Chrysalis get into an argument when Chrysalis is against Sombra's plan to release the Umbrum, knowing they will cause despair, thus preventing her from feeding off love. Threatened with being turned to stone, Chrysalis retreats with her Changelings. After Princess Cadance and Hope discover what the Umbrum are really like, Hope tries to convince Sombra not to release them. Twilight arrives, having been restored by the other Mane 5 with the power of friendship, and shoots a magic beam at Sombra, but Hope jumps in front of him and takes the hit. Sombra then releases the Umbrum.
In Issue #37, Sombra, now emperor, turns Shining Armor to stone at the command of Lady Rabia in a reference to Governor Tarkin destroying Alderaan. He is about to do the same to Twilight and Cadance when Twilight tells them to do this at the dais where the Crystal Heart is so everypony can see it happen. Sombra goes to take Hope with him, to which she reveals that her supposed destiny of becoming a princess did not come true because she left her training to look for him. Sombra was affected by the realization that not only was the Crystal Heart's prediction not true, but she chose him over becoming a princess. When the "rebels" attack the Umbrum, Rabia tells Sombra to eliminate Twilight and Cadance and Hope takes out the Crystal Heart. Rabia tells Sombra to smash it, but instead he unleashes its power, re-imprisoning the Umbrum and saving the Crystal Empire. Sombra is disintegrated by the Crystal Heart's power, but before he can be completely destroyed, Hope saves him with her magic, and together she and all four princesses restore him to physical form, turning him into an actual pony similar in appearance to his mirror-world counterpart. Together, Sombra and Hope decide to restore Princess Amore.