IT'S FUCKING NOTHING
The Promised Land was a plot point set up in Series 8 of the show. Also known as the Nethersphere, it was supposed to be heaven or some other afterlife where all these people the Doctor let die kept ending up.
In Dark Water/Death in Heaven it turns out it was all a simulation in a hard drive being run by the mysterious Missy with Seb and Doctor Chang helping out. Alright, I guess that was a pretty clever reveal, so it wasn't fucking nothing. Bad show on my part.