Complex sentence: In the books the Shayol Ghul is entirely a real place, but the Pit Of Doom, while it lays beneath Shayol Ghul is either partially or wholly not in the world and is the place where the Dark One's power over the pattern is greatest.
Split into simpler sentences: In the books, Shayol Ghul is entirely a real place, but the Pit Of Doom, while it lays beneath Shayol Ghul, is either partially or wholly not in the world. It is the place where the Dark One's power over the Pattern is greatest.