What does Heathcliff reveal about his future plans to Nelly in Chapter 29?

Nelly begs Heathcliff for Zillah's place at the Heights, but he will not let her come. Heathcliff, with a triumphant smile, tells Nelly about his plan to be buried next to Catherine when he dies and confesses that he went crazy after she died and partly dug up her grave, so desperate was he to be with her.