What event does chapter 12 foreshadow?
There seems to be two things that this chapter could foreshadow. First, and most significantly, it foreshadows the scenes at the end of the novel in which Heathcliff wanders the moors, supposedly with Catherine's ghost. Catherine suggests this herself, babbling in her delirium, "they may bury me twelve feet deep, and throw the church down over me, but I won't rest till you are with me. I never will!" Even though Catherine has married Edgar Linton, she still feels a deeper connection to Heathcliff than to her own husband. She believes that she and Heathcliff must be together after both of their deaths. She also correctly intuits that she will die before Heathcliff.
Second, later in the chapter Nelly finds Isabella's dog Fanny hanging from a tree in the garden, a deed Heathcliff performs. This horrible treatment of Isabella's dog foreshadows Heathcliff's terrible treatment of Isabella after their marriage.