Some people say these are precursors. Given that the precursors are actually flood (and the flood are not actually inherently evil), I'd assume that it is not the flood. Also just look at them. Do they really look like flood? If they ARE the flood, the flood would ignore us unless humanity did something to piss off the flood like be evil. They should just leave us alone now. However in the primordial series, 343 guilty spark said he knew where the librarian was. Clearly there was some sort of forunner artifact on the chest plate of the blue winged thing. Given that chief is headed into a dyson sphere (that’s the weird purpley glowy thing in the trailer), a logical guess would be that something is within it that the forerunner wanted to contain. Possibly of forerunner origin, though I doubt it. 343 industries says that this will be WORSE than the flood, so if the forerunner had this new enemy contained, why couldn't they use it to kill the flood? I'm guessing an ancient species which the flood deemed unworthy to uphold the mantle, but the flood did not want to destroy. Alternatively it was too powerful for the flood to handle, so they locked it in a dyson sphere. That is my best guess given the information I have now, and the fact that the flood, despite claiming that its ultimate goals are noble, is quite willing to commit genocide upon an entire species. It is a species even the flood could not defeat. If this is correct, the life form is using forerunner technology, due to the thing on its chest plate.
Alternate possibility- There is a forerunner artifact on the new enemy’s chest, suggesting forunner origin. We know the librarian would be involved, and considering her influence and intelligence, it wouldn’t be hard to imagine that she had her own dyson sphere and sun within for energy. This would also explain how she survived this long. The librarian, aka, “the Lifeshaper” may well have made some combat based cyborg life form. It’s not impossible that she over the course of thousands of years developed something so intelligent and powerful it actually defeated her. So, new enemy is likely forerunner based, capable of defeating forerunner technology, and very smart.
Also take into account the idea that the librarian is in control and has sicked these things on us because she is pissed at us for some reason. Reasons could include insanity, us killing the flood, us destroying halo rings, us killing guilty spark, etc.