I found this interesting article, this is not my thoughts or opinion..but a interesting read and deciding to quote him.
So what did you think of the campaign reveal? It's pretty exciting stuff, eh? I imagine there are a lot of people left a little confused by it so I'll attempt to pool everything we know so far. The following will contain spoilers for the books Halo: Cryptum, Halo: Primordium and Halo: Glasslands, if you're planning on reading those books before Halo 4 then maybe don't read on. If you've already read them or aren't planning too then by all means read on.
The Forerunner books introduced three main characters who may well show their faces in Halo 4.
The Didact (pronounced Die - Dact)
The Didact was the commander of the Forerunner armies a the time of the original Flood outbreak and lead the Forerunner's fight against the Flood. He's a pretty cool guy and generally likes humans.
The Librarian
The Librarian is the wife of the Didact, she is a "lifeworker" and was responsible for managing ecosystems and species dominence on various Forerunner worlds. She built the Library (from Installation 04/Halo CE) and also built the Ark (from Halo 3). She was responsible for the re-population of the galaxy following the firing of the Halo rings. This was covered in Halo: Legends if you've seen that.
The Master Builder
This guy is a bit of a c***. The Master Builder is considered the highest "rank" in Forerunner society. He commissioned the building of the Halos and also fired them on the San'Shyuum home world. He was later trialed for war crimes.
At this point it's worth noting that we don't know the outcome of any of their story arcs, the third Forerunner book is not due for release until after Halo 4 launches, presumably because their plots are intertwined.
At the start of Halo 4, Master Chief, aboard the Forward Unto Dawn, is pulled into Requiem - a giant Forerunner planet inside an exo-planet. At this stage we don't know why he was pulled in, it could have been at the desire of one of the Forerunners mentioned above, it could have been Cortana's curiosity, it could have been a happy coincidence. Let's continue..
Chief is fighting Covenant?! I thought we'd finished that fight?
Yes, we did. However, in Glasslands we learn that following the Human-Covenant war ONI spent a great deal of effort trying to stir up a civil war amongst the Sangheilli (Elites), although we won't find the conclusion to this story arc untilThe Thursday War launches in October, it's a fairly safe bet the Elites weren't thrilled with us meddling in their politics. Other theories include space pirates, mercenaries and religious zealots. Whatever angle 343i take with it, it's not that much of a surprise that we're fighting Covenant again.
What's this?
The Xbox Press site confirms it to be a Cryptum - that's sort of like a Cryo pod for Forerunners, except instead of sleeping for a few days/weeks, they sleep for millennia. It certainly looks intimidating, but it's important to note that it doesn't attack Master Chief and nor do we see it attack the UNSC Infinity (That's the massive ship you see crash landing in the campaign demo).
My theory - and it's only a theory - is that this Cryptum belongs to The Didact and he's not an enemy. The demo is clearly set up to make him look like an enemy, the colour scheme of the Cryptum, the imposing way it rises up behind Chief and the fact that all hell breaks loose right after he arrives would all lead you to think that he's the new “Big Bad”. If it is The Didact, then having him attack the humans would be a considerable character retcon from what we learnt about him in Halo: Cryptum. We also see him scan the Chief and had he wanted to, he could have crushed him there and then. I believe that 343i are trying to make us think he's the enemy, as a prelude to a later reveal that he's on our side. Finally, when the Elite is charging Master Chief with the energy sword he is vaporised by one of the Promethean Knights, had they been looking to kill the Chief it would make sense to let the Elite do it for you – or at least weaken him. We never see that weapon fired against Chief for the rest of the video. I believe they were trying to capture Master Chief alive.
Who are "they"?
We see three new enemies in the campaign trailer. Cortana tells us in the trailer that they're not bilological so presumably their some sort of AI/Sentinels we've not seen before.
The Promethean Knight
From what we can tell these are the leader class, quick and agile in combat and tricky to kill. They also get pretty freaky up close..
The Crawler
Fodder class, seem very much in the vein of the Grunts, quick to kill, but intimidating in numbers.
The Watcher
Support Class, we see them catching and throwing back grenades as well providing shields for the Promethean Knights. They generally seem to hang back and avoid combat. A bit like Covenant Engineers?
In the live action intro we see The Didact's symbol on the screens of the UNSC Infinity, we also see the same "scan" effect that the Cryptum later uses on Master Chief.
I have a theory as to how we got to this point.
Master Chief lands on Requiem, awaking The Didact. The Didact realises that Master Chief has “stirred an ancient evil” (The Master Builder, perhaps?), in order to assist Master Chief, The Didact uses Forerunner slipspace technology to teleport the Infinity (humanity's largest and most powerful ship) to Requiem before pulling it inside. In the trailer we see the Captain of the Infinity looking out of the main viewport towards Requiem, there is a definite confusion/apprehension on his face, leading me to think they could have just been teleported there - that's total speculation though.
The Didact being responsible for the arrival of the Infinity would explain his symbol appearing on the monitors as well as his eagerness to get to the crash site.
The Forerunner infighting between The Didact and The Master Builder in the Forerunner books would tend to suggest that we might be joining sides with some Forerunners against other Forerunners. It also allows for more political sub-plots which have always been a staple of the franchise.
In short, I think that The Didact will fulfill a similar role to The Arbiter in Halo 3 - Hopefully we may get a chance to play as him too!
I'd really recommend swotting up on your Halo fiction before Halo 4, well worth reading:
Halo: Cryptum
Halo: Primordium
Halo: Glasslands
http://www.littleenglishhaloblog.com/2012/06/halo-4-campaign-what-we-know-so-far.html