During the choice available right before you escape from the mysterious alternate world, if you pick the choice that doesn't involve returning to the real world, you wind up with this ending.
Ib finally finds the gigantic painting connected to the real world, but she hesitates to jump into it out of worry. When she does, the picture closes over, and everything is enveloped in darkness...
This is one of the bad endings.
The contents are a little different depending on the condition of Garry and Mary at the time, but the end result is the same.
From ver1.04, there is a new version of this ending.
At the very depths of the New Game Plus dungeon, if you take a certain special action, you will arrive at this ending.
In the end it's no different from the other versions in that it's a bad ending where Ib doesn't escape, but you may still want to give it a look anyway.