Sangheili Major: "Brute ships. Staggered line! Shipmaster, they outnumber us, three-to-one!"
Rtas 'Vadum: "Then it is an even fight. All Cruisers, fire at will! Burn their mongrel hides!"
Yup, Forerunners. The fate of the Didact is unknown as of now, and the Librarian is dead. There are likely more Forerunners still living within shield worlds outside of the galaxy.