Complex sentence: He also reveals that he has snuck other Death Eaters into Hogwarts via the broken Vanishing Cabinet that Montague was shoved into in Harry Potter and the Order of the Phoenix, and that the other cabinet was at Borgin and Burkes.
Split into simpler sentences: He also reveals that he has snuck other Death Eaters into Hogwarts via a Vanishing Cabinet. He had the broken Vanishing Cabinet that Montague was shoved into during Book 5, and the other half of the Vanishing Cabinet was held at Borgin and Burkes.