Cortana died, but I have the feeling that Halo 5 is about trying to bring her back, and it sounds like it may even be possible to bring her back as a living being. In the meantime, we'll probably have to deal with another AI.
Im also pretty sure that the Didact died. However, I dont remember exactly what the captain said, but he made it sound like he already new of other threats, seemingly of the Didacts level.
If you want to see a part of the Chiefs face at the end, you'll need to beat it on Legendary...or search youtube.







