It's amazing here in Egypt. Bill's taken us around all the tombs and you wouldn't believe the curses those old Egyption wizards put on them. Mum wouldn't let Ginny come in the last one. There were all these mutant skeletons in there, of Muggles who'd broken in and grown extra heads and stuff.
Harry potter and the prisonr of Azkaban
"break in" means to enter a place to steal or to interrupt, but I'm not sure if these definitions are correct here.
Does that phrase literally mean "Muggles broke into the tombs"? Then what does "they had grown extra heads and stuff" mean?