....why did Harkon and Valerica assume Serana would be the sacrifice?

Considering how much the two hate each other, it would have been more logical for Harkon to want to use his wife, rather than his daughter.

Also some prophecy, you complete it and the sun only goes dark briefly. Harkon's destiny would have been shooting the sun with a limited stock of blood arrows, then everything goes back to normal in a few days.

This should probably have been posted in Dawnguard Cheats, Hints and Spoilers. It's pretty spoilerific ...

to that end, how do you get a arrow to fly to the sun?

Well Veronica is hiding in the soul cairn. If you ask her to come back with you before the quest is finished, she explains that it would increase harkons chance of completing the prophecy.

Also, the bloodcursed arrows are only arrows in her blood, the prophecy is using all of her blood and bathing the bow itself in it.


And this is the wrong section.

Well, Harkon doesn't even know where Valerica is.

Thanks for the mega spoilers. :/

To be fair, you did click on this.

I meant BEFORE Valerica locked Serana in that crypt and flet into the Soul Cairn. (You guys can understand enough to know it's a spoiler, but not enough to understand the context?)

Valerica did that because Harkon was going to sacrifice Serana. But my question is why she was his first choice when he hates his wife who would work just as well.

Both disappeared roughly around the same, from what I understand, and that Serana was the one who ended up walking in the front door. Also, finding a daughter of coldharbour is probably extremely hard, as only a few out of ~1000 survive the ordeal, so his wife and daughter were really only his options.

In Skyrim General discussion... the last place one would expect DG spoilers.
Anyway, it's been moved to DG hints n spoilers so it's all good.

Again....

Valerica put her plan into motion because Harkon was going to sacrifice Serana. It was to protect her daughter more than it was the Elder Scrolls.

But back then, Harkon still had his wife to use, so why was Serana his first choice when he and Valerica hate each other? He could have detained her, fulfilled the prophecy, and been home for a glass of blood with little problem.

There is no conceivable reason for why his daughter that he loves would be his first choice instead of the wife that he hates, who hadn't disappeared on him yet,

Pretty sure the fact they went into hiding at the same time means that Valerica DID fear being sacrificed, possibly as Harkon's first choice, and hid her daughter so she wouldn't be killed once she was the only available daughter of coldharbour.

Furthermore, when confronting Harkon on the vampire side of the DLC, there should have been an option to say, "Hey Harkon, your wife is hiding in the Soul Cairn, the portal is open and you can go through any time. Use her and spare Serana."

Sad to admit, but Dawnguard is full of plotholes.

Like the fact that Harkon spent thousands of years pursuing a prophecy that was created by a vampire snow elf who apparently had only been a vampire within a mortal lifetime, not millenia, otherwise his brother would either be dead or one of those phantoms.

Well to be fair, elves in the elder scrolls lore can live for a looooooong time. So it's not that unlikely that his brother would still be alive. I mean, it's unlikely that the dragonborn is the only character who can tell from looking in their eyes if a person is a vampire or not, but the life span part is at least explainable.

There is at least one individual that I know of from the game Morrowind who is millennia old (and not in fact a god) Divayth Fyr ( http://www.uesp.net/wiki/Lore:Divayth_Fyr ). Whether or not it is because he is a powerful mage or because he is an elf, there is a precedent for some individual to reach such an age. On top of that, I do remember some talk of Elves not dying from old age in Elder Scrolls lore, regardless of magical prowess.

I doubt Vyrthur and his brother are that old. And they implied that Vyrthur's "corruption" happened recently.

When Serana woke up, she didn't even know that Tamriel had become an empire seated in Cyrodiil, meaning she was locked up sometime in the first era, before Tiber Septim even came around.

Even with magic, a mortal elf shouldn't be able to live close to 3-5,000 years.

Like I added above (perhaps you didn't see my edit) Divayth Fyr was a mortal elf born in the first era according to the wiki I linked, and is an integral part of Morrowind's plot taking place only 6 (I believe) years before Oblivion's. So for an individual to survive so long in the elder scrolls universe is not so unheard of, even for mortals.