...have a tendency to go awry in the most unexpected of ways.

When the Crystal Heart's power destroyed King Sombra, all of the Empire rejoiced. Little did they know, they had played right into the ancient tyrant's hooves.

Having over a millennium to plan, Sombra knew that if a pony managed to get past his traps before he could reconquer the Empire, their was little chance of him surviving. So, he let himself be 'killed', in order to get closer to his true goal undetected; the Umbrum trapped beneath the Empire.

Hiding as just his horn, Sombra laid low in the very heart of the Crystal Palace, biding his time, until the entrance to the underground section was opened by the new Crystal Princess and another princess with a purple coat and familiar magic. Recognizing that magic as that of the pony that foiled his most recent efforts, and knowing of only one princess-to-be with a coat that color, Sombra swears to make Radiant Hope pay for moving against him once again.

Using his magic to reach the deepest depths beneath the Empire, Sombra reached the sealed Umbrum undetected, and reformed his body. Having devised a way to free them during his own imprisonment, Sombra began so, fulfilling his goal even without controlling the Empire. Bursting free in a wave of shadow that left no doubt of their return, Sombra gives Cadence a message for Celestia before following his own subjects into the night:

“Give this message to the Two Sisters. Deliver unto me the princess that yet again hindered my efforts to free my people to me within a week, that I might exact my revenge against that traitorous Radiant Hope, or I shall show all of Equestria why the Umbrum were so feared!”

However, when a week passed, and Sombra found out that the princess in question was not Hope, but rather a new princess named Twilight Sparkle, he finds his plans for revenge derailed. Bound by his word, when she offers to go with him to prevent a war, he finds himself with a 'prisoner' against whom he holds little ire.

And as time passes, Sombra begins to see that he and his 'guest' have more in common than he initially thought.

AU as of Chapter 34 of the 'Friendship is Magic' comics.