Shadow Prison

The Shadow Prison is a prison located in the Frozen North, within the bounds of the Crystal Empire, which serves to contain the Umbrum.

History
After being subdued by Princess Amore, the Umbrum were confined within the Shadow Prison, a timeless limbo from which they were unable to escape unaided. There were two points of entry to/exit from the prison: a red crystal in the Arctic Wastes, and a door hidden beneath the Crystal Castle. The door could only be opened from the outside, and being under the castle was well protected. The crystal allowed the Umbrum some level of interaction with the outside world, but they were unable to pass through it to reach the outside without drastic measures.

Such measures were employed in order to send Sombra, a young Umbrum who had been transformed into an Umbrum/ hybrid, through the crystal. This was done with the intent that Sombra would act as the agent of his people’s escape. After interacting with his mother through the crystal, Sombra was all the more motivated to do so, as the Crystal Heart-which only banished the other Umbrum to the Shadow Prison-could do him serious harm. With his Umbrum powers reawakened through the crystal, Sombra overthrew Amore and seized control of the Empire.

King Sombra, as he had dubbed himself, soon found the chamber containing the door to the Shadow Prison, and converted it into his personal study. However, he refrained from open the door and releasing his people, fearing that he would be no more at home among them than among his Crystal Pony neighbors-turned-slaves. The chance was soon taken away from him, as and  attacked and defeated him, imprisoning him elsewhere beneath the Frozen North. Having anticipated that his childhood friend Radiant Hope would alert them, Sombra cast a spell that caused the Crystal Empire to vanish fir a thousand years.

Hope, wracked by guilt over what she saw as betrayal of her best friend, eventually came across the crystal and interacted with the Umbrum. Deceived into believing that they were kindly, unjustly imprisoned beings, she passed through the crystal and spent the next thousand years in their company, awaiting the Empire and Sombra’s return. After Sombra was defeated by the new rulers of the Empire and their allies, Hope emerged and traveled around Equestria seeking allies against these foes. With a restored Sombra leading them, this group achieved victory, and Sombra prepared to free him fellows at long last.

Hope was then confronted by, and explained her reasoning for aiding Sombra and his people. Hope then brought Cadance to the crystal and into the prison to introduce her to the Umbrum, who continued to disguise their true nature. However, Cadance’s magic revealed the Umbrum for what they truly were, and their leader Rabia dropped all pretense of kindness. After being berated for her actions and realizing that she had been fooled, Hope fled the prison with Cadance to stop Sombra from opening it.

The two confronted Sombra, who confessed his reasons for not opening the door previously. Unfortunately, burst in to attack him and injured Hope, prompting the wrathful Sombra to free his people. However, after seeing the Umbrum at work and realizing his path was his own to choose, Sombra repented of his deeds. He returned the Crystal Heart to its place, banishing the other Umbrum back into the prison. Through the efforts of Hope and the Four Princesses of Equestria, Sombra was then transformed fully into a Unicorn, insuring that he would never again suffer harm from the heart.

In other media
In My Little Pony: Magic Princess, the Shadow Prison is a residence in the Crystal Empire. It's where the Cute Umbrum and True Umbrum live.