Halo 4 and Halo 5 had a couple of series of books used to tell the story of Infinity as well as some background regarding the Diadact. They all came together to give the player/reader more of an understanding of the reason certain people were where they were and how these new ships and enemies existed. Now in the Forerunner saga of books by Greg Bear it is explained that 343 Guilty Spark is actually a mind of a human infused into an artificial intelligence and that the Diadact is angry at humanity due to the Human-Forerunner war caused by the flood. Now the reason explained in the books for the Diadact being still furious with humanity and the reason for the flood, has to do with a race called the Precursors, to whom predate the Forerunners. This race may have somehow been or created the flood and used something called neurophysics to construct all of its impossible long lasting structures including the Domain; a repository of information that wirelessly links the galaxy to help uphold the mantle. This would have calmed the Diadact over his years in the cryptum and made him come to a better understanding of his anger and/or dissipated it entirely, but the Halo array was fired at the end of the novels, which destroyed almost all evidence of the Precursors and the Domain itself at the end of the last book. This all made perfect sense when he came out of the cryptum in Halo 4 and immediately tried to wipe out humanity. This means the books conclusions were directly accepted by 343 Industries and put to use in the making of the game.
In Halo 5 though, the next game in the saga, Cortana is somehow alive in the domain and the whole game is about getting to her. This makes absolutely no sense and I would love if someone could explain it, because this has almost completely tainted my love for Halo. This and that awful take on Halo the fall of Reach.