The King's Glorious Return

"Surrender the throne back to me, there's no hope left for your little kingdom now... Otherwise I'll let my new ally makes sure you trully get thrown into oblivion"

In a different line, Sombra was defeated and seriously weakened, but wasn't killed. The blast of the "explosion" caused by the Crystal Heart rather vanished him to a land lost to time and memory. Similar to the fate he once decided to give the very same empire he tried to take back.

In this land, Sombra would eventually find an unicorn like never seen before inside a colossal cave. He was huge, bigger than whatever he had seen before, probably as big as the castle he once lived in. Trapped in a prison made of a material similar to ice and crystal. And by his looks, he got trapped during a fight. And his cell was surrounded by statues of the beings that trapped this threat in a different time.

Using his weakened powers, Sombra was able to slowly free the mysterious unicorn he had found. And after months of work, this titan, an old bringer of destruction, was unleashed in equestria.

Luckily for Sombra, his act granted him the loyalty of the colossal unicorn. And after each learned the story of the other, Sombra struck a deal with him. If the massive unicorn helped Sombra in taking back the crystal empire and conquering all of equestria, he'll make sure nobody stepped in the middle of the unicorn's desires. And with such promise, both set path to Sombra's goal.

And as they headed back to the Crystal Empire, Sombra got to witness the true power of this unique unicorn. As while riding on his nose, he saw the unicorn getting way bigger than he already was. Eventually relishing on the power his loyal ally wielded when he realized the colossal castle of the crystal empire now seemed like a mere toy under the unicorn's hooves.

Since then, something was clear, defeating Sombra would be nearly impossible now.