Alright, on my PC rather than my phone, so I guess it is time to get serious. *sigh*
What corroborating evidence?\
Geldauran's Claim. Geldauran's Claim is a note written in ancient elvish by a being who refers to themselves as Geldauran and who opposed the Evanuris. Said being was additionally forgotten (intentionally or otherwise). This is first hand evidence that there was a being named Geldauran who fits the traditionally depiction of the Forgotten Ones as enemy of the Evanuris and who the term "Forgotten One" applies to. Thus, there was someone or something named Geldauran that could be be described as a "Forgotten One." This corroborates the traditional account of the Forgotten Ones and the account that listed Geldauran as among their members. Details may be wrong, such as the Forgotten Ones may have been the good guys (or both sides may have been evil), but that is irrelevant to the fact that we have first hand evidence that Geldauran existed, opposed the Evanuris, and was forgotten. There is no evidence that he was among the Forbidden Ones.
So, they are seekers of truth with great arcane knowledge, they know about the Fell Grimoire, they know that Xebenckek is a Forbidden One, they know the names of the Forgotten ones, they have been sent there specifically to find if there is a link between them and in the end one of them calls Xebenckek a Forgotten one but they do not knew what they are talking about.
Seems strange.
That one doesn't even know he is being killed by Xebenckek. He doesn't even recognize Xeb as a desire demon. Let's be clear here, the only things we know about the Band of Three are the codex entries from the Enigma of Kirkwall and the minor note in WoT2. The note in WoT2 was not written by them and has none of their findings. So all we have on their findings and what they know is what is written in the Enigma of Kirkwall. They do not mention Xeb, Gaxkang, Imshael, or the Formless One at any point in their notes. They do not mention the Forbidden Ones as a group at any point in their notes. The Forgotten Ones are mentioned at two points. The first is an open question of, could they actually exist? The second one is in reference to Xeb, but I am going to quote because the context is incredibly important.
The maddening thing is there is still no answer. But the Forgotten One, or demon or whatever it is, must be destroyed.
Did you notice? Here let me emphasize it to make it easier.
The maddening thing is there is still no answer. But the Forgotten One, or demon or whatever it is, must be destroyed.
"There is still no answer" and "or whatever it is." He does not know what Xeb is. He expressly is stating that he doesn't know what Xeb is, only that Xeb must be destroyed. The supposed expert on the Forbidden Ones and the Forgotten Ones DOES NOT RECOGNIZE XEB! There is nothing indicating that any of the Band of Three recognized that they were facing Xeb. There is no evidence. They do not refer to Xeb by name. I don't know how to make this any clearer. That note is written as the person fears for their life and the person has no idea of what is killing him. He doesn't say Xeb the Forgotten One, he says the Forgotten One or whatever that is. He simply does not know...and I pray repetition helped make that point.
That's your assumption, if we must stick to text the Seeker tought that the Demon he was facing (and he knew that it was Xebenkeck) could be a Forgotten one.
And your assumption is that he knows that he is being killed by Xeb. There is nothing that suggests that the person who wrote that note knew that. All the person knew is that it needed to be killed with fire, and it may be a demon...or something else.
That's untrue, he identified Xebenkeck, he knew about the Fell Grimoire, and he knew the lore about the Forbidden Ones.
Besides it is officialy stated that Link between the two groups can be found in other texts.
Where? Xeb's name does not appear anywhere in the Enigma of Kirkwall or the note about the Band of Three in the WoT2. There is nothing suggesting that the author of that last note knew that that demon was Xeb. And what other texts are you talking about? Where is it stated that these texts exist? Who wrote them? What knowledge did they have on the subject? I won't say that there is nothing suggesting that those texts exist, but I recall nothing on the topic, and without knowing details about them, it is difficult to have any sort of conversation about their validity, since the possibility exists that they were written by someone who was not familiar with the topic.
If you are talking about the quote from WoT2 that states "They were bound with holy vows to uncover what lore they could, to trace the roots, and return that lore to us. In their search- why there are so many blood mages in Kirkwall, the nature of the Veil there, whether the Forgotten Ones in Elven lore are connected to the Forbidden Ones in other works." You are misinterpreting the last clause. What he is writing is, they were investigating whether the Forgotten Ones in Elven Works are the same beings as the Forbidden Ones in works by the TI. He is not saying that there are works that say they are the same.
And after that is said that they found invaluable tomes about the lore they were seeking.
They found invaluable tomes. It does not say that the tomes were on topic for what they were necessarily searching for (after all the tomes were found in what they believe were probably the visitation chambers for the Archon or something of similar importance). Even if the tomes they found were on point, they were sent to multiple topics, and further a tome saying "hey these two groups are different" would still be an answer to their quest. They mention no discovery of anything on topic, so we have no evidence of what those tomes might have said on any of those topics (other than their discovery that people were intentionally thinning the veil at Kirkwall).
So it's a very long shot to think that they didn't knew what they were talking about.
Except for the fact that he expressly states that there is still no answer and he doesn't know what is killing him. >.>