The Dark World is the world into which the dead things from all previous/other worlds are dumped. This is the world that John says he was bound to the book in, and the world in which he spent countless lifetimes.
Description[]
John is surprised by the trees in Arkham, which implies that the Dark World has a distinct lack.
The Dark World "thrives on fear", the land itself seeming to exist to only cause pain. It "feeds those with a voracious mind".[1]
Part Thirteen - when clarifying who John actually is
Part Fifteen - when asked where everyone goes after they die
Part Sixteen - when discussing the fear of the unknown and the fear of the unknown consequences of a decision
Part Twenty - when John chews on how he changed after the hospital and Lilly the nurse
Part Twenty One - when Arthur threatens to send Yellow back there if he doesn't cooperate
Part Twenty Two - after Arthur reveals the Glass of the Cana can't send Yellow back to the Dark World
Part Twenty Seven - when considering who they are and how they got there
Part Twenty Eight - when Arthur talks about meeting up in the Dark World
Part Thirty - when thinking about why the memory of the Dark World lingered with John, and when conferring about the Vanguard
Part Forty - when Kayne finally reveals what happened to John before he came back to Arthur
Part Forty One - when carefully talking about John's second time in the Dark World
Part Forty Two - when creating Yorick he mentions he exists in this world and in the Dark World
Part Forty Three - when John speaks to the witch about his experience there
Part Forty Four - Arthur wonders if he's in the Dark World but is informed he's in the Waylay. John mentions that Arthur would know whether he was in the Dark World.
Part Forty Seven - Yorick has Arthur create a Hand of Malevolence - an object from the Dark World.
Part Fifty - It was revealed that Alia is an Ardor - the human connection to a Vanguard
Part Fifty Two - Kayne mentions John's time in the Dark World and references sending Arthur and John there.