How does Mrs. Dean describe the changed appearance and demeanor of Heathcliff prior to his death in Chapter 34?

Nelly goes to check on Heathcliff and she finds him leaning on the open window, staring outside. When Nelly moves to close the windows, she sees Heathcliff's frightful face and runs for Joseph to make him attend the master. Nelly gives in to wild fears that Heathcliff is some kind of ghost or monster.