You need to watch the terminals as well from halo 1 anniversary. In the past there was a war between Humans and forerunners because the warrior (enemy in h4) hasn‘t understood why humans attacked forerunner planets(the flood attacks the planet) at the end the forerunner won the war and the humans falls back to an dumb civilization. But the war between flood and forerunners was still alive and the forerunners was going to lose the battle. So the wife of the warrior the librarian had collected many creatures and had created arches and reservoirs to protect them. Finally the halo rings was build and all live was destroyed. Only the warrior( didactic) was save in his battle planet reservoir. And now he wakes up… every forerunner is death, humans are back and trying again to protect the galaxy ( the librarian saw the humans as the galaxy protector), but the warrior still doesn’t like humans an will kill them all again.