Pyramid of Shadows

The Pyramid of Shadows
The Pyramid of Shadows is a bizarre extra dimensional space It exists beyond space and time, appearing in multiple places in the world and planes beyond. The space within it knows nothing of the passage of years. It’s a prison, designed to keep whatever is trapped within it from escaping. The interior and it's features are mutable and adapt to requirements of it's prisoners. 
Background
In centuries past, a tiefling wizard named Karavakos made a fatal bargain. The twin threats of monsters of the wild and rebellion from within threatened his iron grip over the lands of his petty kingdom. In desperation, Karavakos forged a blood oath with the demon named Orcus, and asked for aid. A legion of undead soldiers came to march under his command, and he stepped up his efforts to restore order—and a harsh justice—to his domain.
 The legion of undead came at a very reasonable price. They would fight on his behalf and obey his orders without question. In exchange, Karavakos reveal a secret that Orcus had been seeking for centuries. Defeat of his undead army, however, would be met with unfathomable punishment. 
 Cautious and conniving, Karavakos also made a second deal, behind Orcus's back, with a powerful devil. Though this devil's true identify has never been revealed, it supplemented Karavakos's already formidible forces with infernal warriors. 
 Only an error made by the wizard could result in the defeat of this force of undead and devils —no army in the world could defeat them on the field of battle.
 Karavakos led his legions on one conquest after another. What had been a small kingdom grew into a mighty nation to rival the great human empire of Nerath that held sway over much of the land at this time.With winter’s arrival, Karavakos called a halt to his campaign of expansion. He returned to his palace with the eladrin princess, Vyrellis, who was to be his bride. Together, he proclaimed, they would rule a new empire, and with spring’s dawning his armies would march into the Feywild, extending their rule across two worlds.
 When spring came, his armies marched through the thin border between worlds. In the Feywild, though, the legion of devils and undead met their match. Invincible on the battlefields of the natural world, the infernal warriors had no such protection once they stepped into the Feywild. Though Fey spears and arrows proved potent against the legions, still they would soon overwhelm the Eladrin forces that were dispatched to stop them. The eladrin known as Sharrin the Crone, was seen meeting the hordes of undead and devils with the words of the Raven Queen, but even her magical powers could not overcome the invaders. Desperate to save her home and her people the Crone called upon powers best left undisturbed. Through a grueling ordeal lasting several days, she summoned the power of shadow. When the ritual was over, no creature was left alive in the vale and Mross-Kagg. Plants and animals alike
were all dead, so were devils, elves and the undead hordes. There were no remains to even suggest that a battle had ever occurred, and no sound of any creature could be heard anywhere. Only Sharrin escaped this fate, but she was no longer the person she once was; the ritual had irrevocably changed her.”
And so the undead and infernal forces were destroyed, putting an end to Karavakos’s dreams of conquest—and severing his diabolic bargain. Karavakos was imprisoned in the Pyramid of Shadows, his eladrin princess at his side, but far outside of the grasp of Orcus. 
 The Pyramid of Shadows is one of many infernal prisons created to confine powerful beings. Just like a prism splits light into its component colors, the pyramid split Karavakos’s life force into free-willed splinters, each one containing a fragment of his power. With his power spread among various splinters, Karavakos couldn’t hope to break free from the prison. Enraged, he lashed out at the only person he could blame for his failure—Vyrellis, who he claimed tempted him into his ill-fated attack on the Feywild. He cut Vyrellis’s head from her shoulders and threw it into the charnel pit at the heart of the pyramid. But Vyrellis didn’t die. Her life energy, too, was splintered upon her arrival in the pyramid.
 After centuries of imprisonment, Karavkos lured the group of heroes, later to be known as the Godsworn, into the pyramid of shadows. Ultimately, they played into his hands, defeating each of his aspects and finally freeing him from his imprisonment. 
 Now, the pyramid of shadows floats empty, outside the realm of the material world, waiting for new denizens.