What does Mortimer tell Harry about the being he is pursuing?

Mortimer explains to Harry that the barrier between the worlds is thin, almost like Saran Wrap, and that recently, it has become weak and flimsy. It now lets more spirits through, and the ones that come through are stronger than before. He explains that this weakening of the barrier may have been what led to the spawning of the being Harry is following; all Mortimer knows is that it is called the Nightmare.