I'm guessing by that comment, you've not played TLJ or Dreamfall: TLJ. Brian went through the divide in the early 1930s and was trapped there for around 200 years. I think it's actually more that 200 years passed for everyone else, but only hours or days for him until he and the Undreaming joined. None of the games give explicit answers to how long into his time in the divide before the Undreaming - though it appears to be instantly. We do know he is aware that a very long time has passed, but how he came upon that knowledge is unknown, as is how long he perceived himself to be in the Divide.
Though he was once a normal man, we've no way to know what he truly became. I suspect it was being trapped there that drove him insane - as well as dealing with the Undreaming, of course.
I did play both TLJ and DF but you're completely correct I switched times in my coment sorry.
But still, those powers are given to him by the undreaming (or at least that's what we suppose from book 5). I don't think he develops all those powers from any other source althought it may be possible.
Actually, in TLJ Brian appears to be a normal man, friend of Chavez, trapped in Arcadia who is unable to come back to Stark. And still that same man has passed through all those things we saw in the sequels: Travelling to the Tibet, being saved by Cortez, teleporting to the divide and being "attacked" by the Undreaming. And I don't know if that's beacause the RTG team didn't know yet what to do with Brian or because he was fighting against the undreaming but still was not possessed by it.
I have to say too that I played both TLJ and DF a long time ago, so there is a lot of stuff i don't remeber from those games let alone specific words from some of the characters that may, and probably are, really important.