Limbo

Limbo is the name given to the space between worlds, a state in which time does not pass for those who enter it.

History
After Stygian merged with the Darkness to become the Pony of Shadows, the Pillars of Equestria eventually realized that they lacked the means to put an end to his evil. As such, Starswirl the Bearded created a spell that would allow them to banish him to Limbo, though the Pillars themselves would be forced to travel there with him. Having infused their magic into a Crystal Seed that would eventually become the Tree of Harmony, the Pillars assembled at Ponhenge and enacted a spell to summon the Pony of Shadows. They then enacted the banishing spell, and all seven were sent to limbo, leaving behind the Pillars' signature artifacts in the process.

Starswirl's journal eventually made its way into the hooves of Sunburst, and with the aid of Starlight Glimmer and the Mane Six he discovered what had become of the Pillars. became determined to free the Pillars from Limbo, believing their presence would be of great benefit to Equestria. Aided by the Cutie Map, her friends set out to recover the other artifacts: Rockhoof's shovel, Flash Magnus' shield Netitus, Mistmane's flower, Mage Meadowbrook's mask, and Somnambula's blindfold. By reuniting these artifacts with the journal at Ponhenge, Twilight and her friends were able to reverse the spell. Starswirl, Rockhoof, Flash Magnus, Mistmane, Mage Meadowbrook, and Somnambula were all returned to Equestria...but so was the Pony of Shadows.

Though the Pillars and their adversary had not aged in Limbo, their return to Equestria left them weak, and the Pony of Shadows fled to regain his power after destroying Starswirl's journal and Ponhenge to prevent them from banishing him again. However, after some research, Twilight determined that the Elements of Harmony could be used to banish the Pony of Shadows without sending the Pillars with him, though the Elements themselves would be lost. The map then showed them their enemy's hiding place in the Hollow Shades, and they confronted him in the Well of Shade. During the struggle, Twilight realized that Stygian was still himself within the Darkness, and learned why he had become estranged from the Pillars.

Learning that their friend had not been trying to betray them but to help them, the Pillars joined the Mane Six and their friends in freeing Stygian from the Darkness. The portal to Limbo swallowed the evil mass, but because Stygian had been rescued and not exiled the Elements remained in Equestria.

In other realities
The Pillars of Equestria of the Mirror-Universe intended to banish their own world's Pony of Shadows to limbo, just as the Pillars of the main universe later did to theirs. However, Princess Eris interfered, stealing Starswirl's journal and thus preventing the spell from being cast.