Complex sentence: They see little difference between extreme pleasure and extreme pain and drag Frank, willingly at first, into their extradimensional realm, where Frank begins what he realises too late will be an eternity of torture.
Split into simpler sentences: They see little difference between extreme pleasure and extreme pain. The beings drag Frank, willingly at first, into their extradimensional realm, where Frank begins what he realises too late will be an eternity of torture.