Deep limbo is alike limbo. It is a location outside of the normal universe. If you die within limbo, you are sent barrelling into THIS horrific and torturous place. It is hard to breathe, and it may cause you to shake uncontrollably. Unlike limbo (which has a limit for the amount it can contain), deep limbo has no limit. It will fill itself up infinitely, never ending.
Whereas limbo is accessed by popping down a dimension, deep limbo is accessed by popping down a dimension after THAT. Deep limbo is what Conniving Puffin considers: The 1st dimension. You are practically just a line in here. An unimportant, practically invisible, line (although Conniving Puffin neglects this visually, as they want to show the characters). Go down one more dimension after this, and you're in the zeroth dimension, or in other words, Death. Yes, Death is a place. The zeroth dimension. The very edge of the space-time continuum. Heaven and Hell used to be things, but when alchemy came into play, all souls were sent to be tortured endlessly at the black, blank, edge of the universe.