Who is Tarohne? She is not a big central character of DA2, but she is the first char we are presented who has some understanding of what Xebenkeck truly is.

To put it shortly in context, let’s refresh some quests in DA2

Everything starts in the coast of Kirkwall, where we find Cullen with a Templar recruit called Wilmod who he suspects is meeting escapee mages. He tortures Wildmod until he turns into an abomination, presenting to us a unique piece of lore up to this point: that mages are not the only ones who can become abominations as the Chantry has been telling us so far. Of course, Templars didn't know about it, as usual.

After some tracking of the responsible of the disappearance of Templar recruits, we find Tarohne who despite being a "sort of" unreliable narrator, she puts in context several points we learned or inferred from the codex The Enigma of Kirkwall: with the assistance of a mage, demons can possess anyone: a Templar, a noble, etc. In ancient Tevinter, she says, they could hold demons as allies. This is something that we see in DAI when Corypheus teaches the Venatori how to bind themselves to demons. This is the lore that explains all the plot about the Grey Warden raising an army of demons. I'm not saying the Grey Warden acquired this mastery, since it was altered [every time they bind to a demon, they became puppets of Corypheus], but that plot of DAI allowed us to see that the ancient Tevinter had a deeper knowledge of the demons to control them in similar fashion as Tarohne claims.

Later, we find Keran, a Templar recruit, restrained in a magical cage, tortured by nightmares as, I imagine, the demons keep pushing him in order to accept the possession. 

This situation immediately brings to my mind what Flemeth said to Morrigan: "a soul cannot be forced upon the unwilling". This is a lore deeper and truer than any Tevinter knowledge we can get in DA series, since it comes from one of the first Elvhenan: Mythal. So I’m going to assume this is the true statement to understand what’s going on, lore-wise. 

It's never explained explicitly what happens if a soul is forced upon the unwilling. I can speculate two options:

It may produce the destruction of the souls, since it seems to be the procedure that allowed the Grey Warden to destroy the Archdemons, preventing their further rebirths.  

Another possibility is that this is the case when abominations occur and demons take total control of the person, changing their bodies into grotesque shapes. 

However, option 2 seems to contradict a lot of situations where we saw a mage accepting possession willingly and becoming a deformed abomination afterwards. One could argue that this is because these abominations were product of accepting the possession of a spirit with "negative” behaviour [rage, desire, pride, etc], while the non-abomination possessions we know in DA series have occurred with gentle, kind, and positive spirits, for example, Flemeth [with Mythal, uhm...I know, she is not super positive as a goddess of Revenge], Wynne and Evangeline [with a spirit of Faith], Anders [with Justice, who didn't transformed Anders even when Justice transformed itself], Sigrid Gulsdotten [with some kind spirit  of teaching for sure]. There are other anomalous cases of possessed people that were un-possessed later, like Fiona and Mihris, but I'm not counting them. They are not possessed anymore.

If we see what Keran says, it seems to show a small contradiction with what Tarohne said: the demons can’t simple take control of the person without their acceptance first. So they try to push and tempt the person in question, torturing and tempting them in dreams, but never forcing the possession straightforward. We can infer that in nightmares, Keran was tempted to accept the possession but he "resisted". If we keep into consideration Flemeth's words and all what I said before, it seems that only through a willingly person the possession [with or without a deformation of the host body] can be performed. Certainly, in all the instances where we saw possession in DA series, we saw demons making deals and tricks to convince the mage to accept them. So Flemeth's words strike true, and I'm inclined to think that the only situation where we see unwillingly possession is when the Grey Wardens kill the Archdemon, and it ends up in a blast that destroys both souls. If this is true, this could explain why the souls of all the previous archdemons were not born again after being slain, as the Avvar and Solas told us that it happens with spirits [Urthemiel can show this with Kieran too].

In any case, when we get rid of Tarohne, and save the Templar Order in Kirkwall from a wild corruption of Templar abominations, we assume this quest has been concluded. However, years later, in the following act, we have this curious quest of collecting "Evil tomes". As we collect them, we obtain the complete version of the codex Forbidden Knowledge, where we learn several interesting things gathered by Tarohne:

She has been studying other mages that made contact with something called Xebenkeck, a Forbidden One. It requires a lot of blood for that.

She claims that the first mention of the Forbidden Ones is in 4:2 Black [1495 TE]. This is after the Third Blight and the Schim of the Chantry between the Chantry of Orlais and the Imperial Chantry of Tevinter. To me, it looks like a very late age for listening about the Forbidden Ones for the first time, specially if we keep in mind that Tevinter have always been in contact with spirits or “Old Gods” before. That this denomination appeared so late makes me suspect that they were named in another way before [maybe Forgotten Ones?].

Her research seems solid because she wrote that the first contact with  Xebenkeck was done in the "deep Fade". We had found a soft connection between the more reliable codices of the Elvhenan:  Exile of the Forbidden Ones and the The Deepest Fade, which makes us suspect that maybe the place of exile of the Forbidden Ones had been the Deepest Fade, and it is there where these first Tevinter magus went to contact Xebenkeck. It’s coherent, at least.

She implies that the Fell Grimoire has the names of the Forbidden Ones and names them: Xebenkeck [DA2], Imshael [The Masked Empire, DAI], Gaxkang the Unbound [DAO], and The Formless One [I bet, in DA:D].

It is stated that it is they who taught Blood Magic to humans. This is very interesting, because Solas claims this is a school of magic like any other [check section “magic” in Solar shares lore: Part 2].  Solas’ comment makes us infer that this was part of the Elvhenan arcane culture like any other magic school, and it may link the Forbidden Ones as part of the Elvhenan society at some point. We also know that Solas never saw spirits and demons as different entities, but the same one with twisted purpose. Considering this knowledge, the main question is why the Forbidden Ones became what they are? What were they forced to do that twisted them so much into what they are now? Was it the war against the Titans? Could it have happened that Forgotten Ones that were forced to the Evanuris and twisted their purpose to become Forbidden Ones? Or the Forbidden Ones are simply another way of being elvhenan, and since they were not forced into shapes like the elves, they remained spirits? Certainly, all of them have less impulsive desires, and can be very civil talking to them [For example, Felassan speaking with Imshael]. Sadly, there are no many answers to these questions, only speculations.

Tarohne claims that Andraste was a deluded fool and the Maker is a hoax, which is a sentiment that other characters who understand this world much better seem to share. Solas prefers to remain silent when it comes to Andraste and the Maker, but his most direct agent we knew about, Felassan, claims that the Maker is a human invention [** check the end of the post for the cite].

She claims that there is more evidence about the Forbidden Ones than the Maker, and we, as player, can totally support this statement.

As we seek all these tomes, we end up in a Forgotten Lair, in Darktown, where we find the copy of the Fell Grimoire made by Tahrone.

The Fell Grimoire is an important book in DA lore, it appeared in the codex The Enigma of Kirkwall, in particular in two sections that imply part of the story around this book:

In the back alleys of Lowtown you can find extraordinary things. Priceless tomes of knowledge can be bought with a handful of gold: The Chant of Archon Lovias, a whole chapter of the Midnight Compendium. Some of these books were thought lost forever!

And these are no forgeries. I've verified their authenticity myself. The fences have no inkling that what they're selling has value. Where did these books come from?

After several failed attempts, I got my answer underneath the city. There is a hive of hidden passages in Kirkwall's sewers. Now and then a lucky "sewer rat" comes across an unlooted chamber, and then a cache of ancient Tevinter relics spreads through the black market. We must search below the city.

—Underneath a cobblestone with curious markings, faintly glowing. It is signed, "The Band of Three"

A recent trove was uncovered. This one was big, perhaps the archon's visitation chambers.

And a flood of tomes is on the market. Even the simple fences know something is amiss—they've raised their prices at the frenzy of collectors. One said he sold a copy of the Fell Grimoire! I doubt he would lie; how could he know that tome is a mere legend?

If that is real, then what of the Forgotten Ones? This journey has taken us to many strange places, and made us re-evaluate many former truths. Where will it end?

—Hidden under a cobblestone with curious markings and signed, "The Band of Three"

So, long time ago, when Kirkwall was Emerius [Kirkwall history and design], an underground network of arcane research was developed. The relics left there reached the surface via the black market when looters explored this underground network and could unlock new chambers. That's how we track the rumour that someone may have got a copy of the Fell Grimoire. This book, somehow, ended up in Tarohne's possession, who made another copy to preserve the knowledge.

The book is supposed to teach how to summon the Forbidden Ones, ancient demons that, so far, we only know they are four: Xebenkeck, Imshael, Gaxkang the Unbound, and The Formless One. Each of them known and fought in each game of DA with the exception of the Formless One [probably they will be present in DA:D].

Now, up to this moment, this is all the information we have about the Forbidden Ones in the games. We have a more broad vision of Imshael's personality in the book The Masked Empire, but there is no mention of the Forbidden Ones  or their history. The only mention in the book is that Celene has studied about them.

However, as we saw in the codex The Enigma of Kirkwall, one of The Band of Three has a kind of a leap in their thinking process and realises that if they found the Fell Grimoire--which was supposed to be a book of legends and myths--now any legend and myth acquire a degree of truth: hence, the Forgotten Ones may also be truth. The last note of the codex straightforwardly connects Xebenkeck with a Forgotten One:

We went to the center of it all. F. is dead and I am alone and injured. I must go back and put an end to it. The maddening thing is there is still no answer. But the Forgotten One, or demon or whatever it is, must be destroyed. I fear one may already be unbound.

I foreswear my oaths. The magister's lore must be burned and the ashes scattered. No good can come of it. And Maker help us if someone does answer what we could not.

—Hidden near curious markings and signed, "The Band of Three"

At some point one suspected this was a typo in a rushed game, since Forgotten Ones and Forbidden Ones are names that can be easily mistaken. However, the World of Thedas book came up with the true intention of that last sentence:

This exchange of letters between Seekers claims information we didn't see in the Enigma of Kirkwall:

Blood magic origin is not from Tevinter but from Arlathan. This is a very interesting piece of information because coincides and makes sense with the information we got about the Joining ritual of the Grey Wardens that faced the First Blight: they created the ritual and consulted Tevinter Magisters and Elves, who shared "ancient knowledge of Arlathan". This is not new for us after all the analysis we did with Murals in DAI, Fen’Harel’s mountain ruins, The Lost Temple of Dirthamen, and  Vir Dirthara, but we can appreciate the lore consistency in the details.

It claims that, at least, is worth investigating if there is a link between Forgotten Ones and Forbidden Ones. It doesn’t straightforwardly says that such link exists, but it’s something given to the player to consider.

As a final detail that I’m not so sure how much to read into it, is that Xebenkeck  drops “Voracity”, a staff which has the same design than “Valdasine”, which is the only curious object we can find in the Vault of the Primeval Thaig.

Voracity has no lore in its description. Its design looks like a dragon’s jaw. There are some spikes of orange crystals that may or may not be a representation of red lyrium. Its stats only show that it’s related to blood magic, which makes sense since Xebenkeck seems to be deeply related to that magic. 

On the other hand, Valdasine, found in the  Primeval Thaig and tied to the codex  Valdasine, is described as “a staff of strange metal, it looks like lyrium and chills one's heart like a remembered sorrow.”  Sadly, sorrow is something that, lore-wise, we can relate to elvhes and titans alike.

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[**] Felassan in The Masked Empire: