My Little Universe: Mindwarp
The Realm of Limbo is where Lucid is currently residing in for his crimes in sending numerous Ponies into a deep sleep, incapable of waking up. Lucid attempted to bring Steven Universe to this dimension, but failed in that task.
It is revealed that the Pillars of Equestria were trapped in Limbo alongside the Pony of Shadows for over a thousand years, in order to keep Stygian from draining more light from Equestria. They all were released from there thanks to Twilight.