Fen’Harel, Old Gods, Creators and Forgotten Ones
Fen’Harel, my prime suspect as the Elder One, who is mentioned in pretty much any installation concerning Dragon Age to date, is known to have tricked the Creators and the Forgotten Ones (presumably the same according to my theories, as will be explained later) and traps them in the heavens (Creators) and the abyss (Forgotten Ones). We don't actually know who or what Fen'Harel is, only that both the Creators and the Forgotten Ones are described as having believed that he/she was one of them.
If we presume that the heavens is the Fade, then the Creators probably got imprisoned in the black city, which as others already mentioned in this discussion, is a prison. So where are the Forgotten Ones? My theory, a common one I assume, is that this is inside the ground of Thedas, and the Forgotten Ones are the Archdemons.
Another part of my theory is that the Creators/Forgotten Ones and the Old Gods are actually one and the same, and that they used to be powerful dragons that roamed Thedas and beyond. Fen’Harel/The Elder One somehow tricks them into splitting their ”bodies” and their ”souls” into two different places (the Fade and the real world), and then traps them there. I doubt that we’ve heard the real story about how this came to be, since the legends seem to be quite obscured by creation of religion (The Chantry) and loss of culture (Elvenen). That means that the body and soul of these powerful dragons are separated on different sides of the Veil, but they could presumably still affect both realms, unwittingly or by outside manipulation.
Fen’Harel, described as a trickster and by Felassan in Masked Empire as someone who always plays the long game, then puts things in motion and waits. For thousands of years, even!
Creating Blights
Why does Fen’Harel/The Elder One do this? In order ot gain power of course. He/she wishes to be a god, the most powerful of all entities, the Dragon of all Dragons, and in order to do this, he/she needs to possess the body of an Old God/Creator/Forgotten One). To realize this plan, he/she invents and uses the Blights.
First of all, he/she makes sure that, as mentioned, the body and soul of all Old Gods are separated. Divide and conquer, etc. The trapped souls (i.e. the Creators) in the Golden/Dark City become mad and corrupted, probably because of their long imprisonment. The Blights then occur when an Archdemon (i.e. a Forgotten One) rises form the abyss (deep in the ground) after being awoken by darkspawn, who are drawn to the taint, described by The Mother and The Architect as a captivating song that unendingly calls to them. This song seems to originate from the corrupted gods. Remember that the body and soul is only separated by the Veil, not physically separated per se.
Creating darkspawn
And who were the first darkspawn? They were supposedly the Tevinter magisters who worshiped the Old Gods and then, through blood magic taught to them allegedly by Dumat himself, entered the Black City bodily. They did this in Kirkwall, a Tevinter City created for this very purpose, as told by Corypheus and codex entries about The Band of Three in DAII. As a side-note, we could actually experience how much the Veil had been permanently thinned all over Kirkwall and how this effected its inhabitants during the course of DAII. Corypheus, presumably one of these very magisters, were later trapped in the Vinmark Mountains by Grey Wardens, fairly close to Kirkwall. Hawke even finds an altar for worship of Dumat there that still works.
The magisters were then thrown out of the Dark City, bringing the taint - the corrupted powers of the Old Gods - with them into the real world, where the taint spread. Humans, Elves, Dwarves and Qunari were all caught up in this, when Broodmothers were created and the generic, mass produced darkspawn appeared and exponentially grew in numbers. The First Blight was a fact.
Enter Grey Wardens
The Grey Wardens, who were possibly reavers consuming the blood by their enemies - in this case darkspawn - accidentally discovered the dangerous but useful properties that it contained, and eventually found a way to use these powers to kill the Archdemons - quite possibly by either desperation or happy accident. Later, Archdemon blood was used, which was even more potent. Remember that Dumat’s Archdemon body was killed twice, giving the Grey Wardens access to the blood of the first archdemon body that had been left behind.
Dumat as the first Archdemon caused the longest, deadliest blight so far, but as the Blights went on, Grey Wardens became more and more powerful and knowledgable enough that they were able to destroy the Archdemons too quickly. The 4th Blight was quelled so completely that blights were believed to be over! And the 5th Blight, which unlike previous blights occurred long after the previous ones, was over in but a couple of years, compared to almost 200 years of terror during the 1st Blight.
We may presume that the Architect was also one of the previously mentioned tainted magisters; he was awoken without memories and through his experiments in trying to stop the taint from controlling darkspawn (by quelling the "song" from the taint), he ”mistakenly” caused the 5th Archdemon Urthemiel to awake. Someone woke the Architect up on purpose, presumably because the 4th Blight had eradicated the darkspawn to the extent that no more Archdemons could be awoken without help. The Architect was awoken by The Elder One for this purpose, but ultimately this plan failed because of the influence of Morrigan and the Hero of Ferelden.
The Grey Wardens must by now certainly be a particularly annoying nail in the eye of whoever wants these Blights to keep occuring. Enter Hawke, who is during his/her years in Kirkwall, the very origin of the Blights, manipulated and/or tricked into freeing Corypheus, who was bound with the blood of her/his own father, Malcolm Hawke. Cory possesses one of two possible Grey Wardens whom he already controls, and thus The Elder One gains a sleeper agent inside the Grey Wardens, the organization that have thwarted his/her plans 5 times already.
We also learn that the Hero of Ferelden, if he/she survives, disappears, as do Hawke. Certainly not a coincidence, for sure!
Andraste and the Maker
Andraste also plays a huge part in this. My theory, as described here (http://forum.bioware...post&p=16681686) argues that Andraste was the soul of Dumat, purified and reborn when the first Archdemon was slain (she was born the same year as the First Blight was finally quelled). This is partly why Morrigan knew about the possibility of Old God Babies in DAO, and how to make sure the soul of the Old God ended up in her own baby.
Andraste allegedly found an entity called the Maker in a dream (meaning that she met him/her in the Fade), and convinced her husband Maferath to march on Tevinter, the nation that caused the blights through the Elder One's manipulation. Kirkwall, origin of the blights, was one of the Cities they tried to take (but ultimately failed to). This suggests that someone tried to undo - or at least resist - what Fen’Harel/The Elder One had caused. Andraste was described as being a slave who eventually was able to command armies, and that certainly sounds like godlike powers to me. If she's actually Dumat's reborn soul, that would explain a lot of what happened then.
Andraste's beliefs were seen as a cult, and the Chantry was created after her death, obscuring the real facts of what actually happened and who/what she was. We’ve seen dragon cults in DAO worshiping Dragons that they believe are Andraste reborn. Andraste also allegedly met the Maker in a dream when she was singing. Lyrium sings. The taint sings. Dragons and songs are common themes connecting Andraste, dragons, blights, taint and darkspawn, as pointed out by others to me in previous discussions. Perhaps whoever met Andraste in the Fade heard her song and recognized her as the soul of Dumat, and Andraste interpreted him/her as a god, despite actually being a reborn god herself.
Andraste and the Inquisitor
In the latest trailer, we see the Inquisitor being granted power by touching what looks like Andraste herself, inside the Fade. If Andraste is the purified soul of Dumat, then it makes sense that she’d still be around in the Fade, and that she’d want to help thwart the Elder One's plans. We also know that Fen’Harel is said by many to still roam the Fade, and we’ve come across several beings that seem to still believe that she/he is still around (Felassan in Masked Empire, the Sareebas in Dragon Age: Redemption, etc).
Perhaps Fen’Harel being in the Fade prohibits Andraste/the Soul of Dumat from doing more than watching from afar, until the Inquisitor is thrown, body and all, into the Fade by an explosion (read: sacrifice) killing thousands. Perhaps, if we follow that line of thinking, this is (as horribly as it sounds) a sacrifice that the Chantry ends up paying, in order to make it possible for Andraste to put a pawn of hers into the world, to prohibit what is happening?
Or perhaps it’s just something that wasn’t part of any plan (as one of the voice overs in the latest trailer suggests), that allows Andraste to reach out and help.
Who is the Maker?
Whoever the Maker is, he/she seems to have a different agenda than Fen’Harel. I’ve seen suggestions that Flemeth is the real identity of the Maker, and not a god at all. It’s an interesting theory, especially as she and her daughters seems to want to resurrect the Old Gods and protect the Dragons. Her own ability to turn into a Dragon also suggests that she may even be the Formless One. It’d also make sense that she knew about Andraste/Dumat’s soul if she was the one who originally encountered her in the Fade.
One puzzling aspect of this is that Flemeth rescues Hawke, who later unwittingly ”helps” the Elder One when she/he accidentally frees Corypheus, who we assume then proceeds to infiltrate the Grey Wardens in Anderfels. I doubt Flemeth of all people would make such a huge mistake. Even Morrigan hints at the end of Witch Hunt, that Flemeth is more than she, and we, thought, and that her plans are great and more dangerous. This could actually be evidence of Flemeth being the Elder One, but somehow I don't think so. I think that we've just not seen enough of Flemeth's real plans to be able to deduce what she's all about.
I mean, Flemeth obviously has her own agenda, as she is the one who rescued the two newest Grey Wardens, both of whom incidentally lacked both knowledge of Blights and insight in Grey Warden customs, as well as were still new enough to the taint that they were able to produce a child in order to trap Urthemiel’s purified soul. She sends them on their way accompanied by her own daughter, thereby making sure that these two are able to slay the Archdemon before even the Orlesian Grey Wardens can arrive at the scene. She's neck-deep in all of this, but presumably not in order to help humans, elves or any of the other races. I think she's trying to restore the Old Gods by any means necessary.
What will the Elder One become?
It’s quite likely that Fen’Harel intends to use one of the two remaining Archdemons (Forgotten Ones) and try to possess the body of one of them, although my guess is that she/he probably originally wanted to possess Dumat’s body in particular, as it seems to have been the most powerful.
Imagine the Archdemons - Forgotten Ones, bodies of the Old Gods - who are powerful enough to ”brainwash” and command whole armies of darkspawn through the song that the taint emits, but still only mindlessly walks across the land, destroying things seemingly without a greater plan or strategy. Their might and the damage they cause is because of how numerous darkspawn are, not because of any particular strategy. This speaks in favour of them being soulless bodies, acting unintelligently (their corrupted souls being trapped in the Black City until destroyed/purified and reborn when the body is slain).
But imagine an Archdemon possessed by a highly intelligent, manipulative entity? Someone like Fen’Harel? That truly would spell the end of the world. Imagine trying to fight an Archdemon that can plan and use strategy.