Something that has been bothering me about Dawnguard has been the discrepancies between Gelebor's backstory and info and Serana's backstory and info, as well as lore from other places in the series.

Consider these:

• Gelebor's brother made the Auriel's Bow prophecy.
• Serana was sealed away to prevent Harkon from fulfilling said prophecy.
• Serana, when asked at the beginning of Dawnguard how long she's been sealed, reveals that she was sealed away before the Empire existed (because she doesn't know what the hell it is). This puts her time of sealing before at least 1E 266 (as http://uesp.net/wiki/Lore:Empire, and I can't seem to find a date for the beginning of the First Empire).
• Serana's family obviously existed before Harkon found out about the prophecy, which makes her even older.
• The Arch-Curate was made a vampire by one of his initates (the reason for him making the propecy), so that happened back when the Snow Elves were not Falmer (or at least, not Falmer in large enough numbers to attack the Chantry, and Gelebor stated that it took them generations to become their current form). So this was before the Dwemer disappeared.
• Ergo, the prophecy was made before the Empire existed and the Dwemer disappeared. Again, the latest year I could get for the First Empire was 1E 266; the Dwemer apparently disappeared around 1E 700.
• Serana and the Arch-Curate both have the excuse of being vampires, but Gelebor does not.
• Even other elves like Dunmer have lifespans of a few centuries at most unless they're awesome mages like Neloth who can extend their lives through magic. Even if Snow Elves were to live a ridiculously long period of time, Gelebor hardly even looks aged!

So uh...how the hell is Gelebor still alive? I suppose it could be Auri-El's doing, except fat lot of good that did for the other Prelates that died horribly (they're spectral as opposed to living like Gelebor). Or maybe the writers screwed up the lore a little bit? Dawnguard does have some interesting...discrepancies between even the events of backstories for characters like Serana.

i always thought he was an avatar of auriel, and that the term "brother" was used religiously. Also i think serena was between reman and septim, not before the allessian empire

Actually, the Construction Kit reveals the empire Serana is talking about is the Septim Empire.

Meaning she was sealed between 2E430, and 2E896, probably closer to 2E896 then 2E430, as she doesn't act like an Empire being in Cyrodiil is anything recent.

Divayth Fyr has been around for thousands of years as well, and he's not a vampire. I would assume that the Chantry's magic is just as powerful as the Telvanni's, if not more.