Dark Dimension

"All things age. All things die. In the end, our sun burns out. Our universe grows cold and perishes. But the Dark Dimension, it’s a place beyond time."

- Kaecilius

The Dark Dimension is a dimension in the Multiverse ruled by Dormammu. It is a strange and hostile universe in which time does not exist in the same way it does within the regular universe. This allows it to be the power source of life-extending spells and the removal of mortality, at times, giving sorcerers superhuman ability beyond their magic. Tapping into it, however, would open one to Dormammu's influence and make it more likely to become his soldier in his thirst for dimensions to conquer, and even if the sorcerer remained pure, the process was still extremely uncomfortable and likely not worth the effort, according to the Ancient One.

The Ancient One's Eternal Life
The Ancient One often drew power from the Dark Dimension to fuel her immortality but forbids others sorcerers of Kamar-Taj from doing the same.

Theft of the Book of Cagliostro
Kaecilius, one of the sorcerers of Kamar-Taj had heard of the Dark Dimension as a place without time where he and the Zealots would be immortal. He knew that the Ancient One herself drew power from the Dimension and believed her to be a hypocrite as she forbade others from doing the same. One night, Kaecilius and the Zealots went to the Kamar-Taj library with the intent of stealing a spell from the Book of Cagliostro to bring Dormammu to Earth. Using the pages from the book, Kaecilius deciphered the spell hidden within it and used it to empower himself and the Zealots, summoning the power from the Dark Dimension and giving them unparalleled control over the Mirror Dimension. The process turned the skin around their eyes dark purple and outlined silver and cracked. Overflowing with these new abilities, Kaecilius began to use the new power to defeat the Masters of the Mystic Arts and destroy the Sanctums to allow Dormammu to come to Earth.

Battle at the Hong Kong Sanctum
Upon arrival in Hong Kong, Kaecilius and his Zealots found themselves challenged by the Masters of the Mystic Arts outside the Hong Kong Sanctum, with the Masters now being led by Wong. It did not take long however for Kaecilius and the Zealots to destroy the Hong Kong Sanctum and succeeded in bringing Dormammu and the Dark Dimension to Earth, killing Wong by trapping him underneath rubble and many other civilians in the battle as the Dark Dimension began consuming the sky.

Doctor Strange and Karl Mordo headed to the Hong Kong Sanctum only to discover it has already been destroyed and Dormammu is coming to Earth. Using the Eye of Agamotto and breaking the Natural Laws of Time, Strange reversed what had happened, returning Wong and many others from the dead despite Mordo's protests. He was interrupted mid-spell, freezing time around the fighting. Kaecillius returned and declared that Dormammu's arrival couldn't be stopped anymore and once again mentioned that the Dark Dimension is "beyond time". Hearing this, Strange figured out a way to defeat Dormammu and headed to the Dark Dimension where time was irrelevant. Strange confronted Dormammu saying he wants to Bargain. Dormammu shrugged off Strange's words and proceeded to swiftly kill him. Suddeny Strange appeared again repeating the exact same words as before. Confused, Dormammu attempted to kill Strange again, only for him to reappear and repeat the cycle over and over.

Strange explained that he had created an endless time loop, and would keep both him and Dormammu trapped repeating the loop forever until latter relented and listened to his bargain. Eventually Dormammu surrended and asked what Strange wanted. Strange demanded he never return to Earth and take Kaecilius with him. He returned to Hong Kong to complete the Time Spell, and watched as Dormammu took Kaecillius and his followers away.

Trivia

 * In the comics, the Dark Dimension is an amalgamation of different realities, that was inhabited by the Mhuruuks, before it was conquered by Dormammu.
 * The overall visuals used for the Dark Dimension were based on Eternity's Realm from a famous panel of Strange Tales Vol 1 #138 where Doctor Strange asks Eternity to give him power to save his master the Ancient One and defeat Dormammu.