The Endless (they have a proper name in the latest encyclopedia - Xalanyn) They're immune to the firing of the Halo Rings. Halo Infinite touches on nearly every major period throughout Halo's history from games as well as books. There are even Keyes flashbacks that expand a little more, they dig a little bit more into Halsey and the creation of Cortana, and so much more.
https://www.halopedia.org/Xalanyn
Halo Infinite is bringing a lot of mysteries that have been hanging out there for years full circle and are ready to reveal some big revelations, which is what I've wanted Halo to do rather than play things safe. A thread beyond the flood. They're digging more and more into mysteries like The Domain. We may learn quite a bit about who ruled the galaxy prior to the Forerunners. Also, it's possible that the guardians are not actually forerunner at all, but could be weapons from a different species. Lots to unwrap, but I like where they took the story and lore. And I enjoy what they did with Cortana's story and the new AI.
It's very possible that the Xalanyn ARE the Precursors or an offshoot of them in some fashion. It was said Forerunners betrayed those that came before them. Perhaps the Xalayn were what came before, perhaps they are the universal caretaker the forerunners overthrew. But their technology is far beyond forerunner tech by the looks of it. They even modified that laser to cut into forerunner infrastructure like butter, something that surprised the new Cortana.