Not much to say this time, because the Soul Cairn...wasn't much of anything.
The Soul Cairn is a endless wasteland, under a dead sky. It strikes me that it fits Harkon's vision perfectly...above you, there is only an empty hole in the sky, forever hungering, and beneath it, a vast wasteland of basalt cyclopean ruins, fissured earth, and dead brush. I doubt that the brush was ever alive.
The air is thin, and the only smell is of dust. Pits dot the landscape, filled with strange white fire, and dark crystals hover over some ruins, but the only sounds are the hissing of the fissures, and cold wind howling through the basalt ruins of stone. Occasionally, one sees a soul, silent and hunched, or one of the Bonemen, Mistmen, and Wrathmen. Soul gems are everywhere.
I explored the area thoroughly. I found spell books to summon the Bonemen, Mistmen, and Wrathmen. I found the three gems of the Reaper, and plundered his house and killed him. I also learned to summon a skeletal horse, named "Arvak".
It had occurred to me to wonder why others, rather than making bargains with the Ideal Masters (the apparent rulers of this dimension), did not simply enter and take what they desired, as I did. But this place is hostile to all life...even I, as an undead being, found that it tried to separate soul and body. For a living thing, it would probably kill almost instantly.
We found Valerica, trapped in one of the ruins. She took an altogether rather confrontational tone with Serana, which I thought was somewhat interesting, considering her plight. On the other hand, Serana did not apparently feel inclined to take that sort of treatment, as she launched into a reasonably devastating diatribe, which I personally thought was justified. However, if I had any doubts about the relative decency of her mother, Valerica quelled them. She apologized, and explained that everything she had done, she had done for her daughter. Words, of course, mean little, but she backed them up by offering to give us the Elder Scroll we needed.
There was, of course, a catch: the Ideal Masters had apparently decided that Valerica's soul was just what they wanted, so they'd trapped her in that ruin, behind some sort of ward. There were three "Keepers" powering it, via several souls per "Keeper". These "Keepers" were arranged around the ruin, probably so that Valerica couldn't get at them. Well, that meant I could, so I killed them.
Incidentally, while I was at it, I met a soul calling himself "Saint Jiub", who drove out the Cliff Racers from Morrowind, whatever a Cliff Racer is. I'd been picking up pages of some book along the way - turned out it was his book, which he needed to complete his "Opus". Fine, whatever. I found a couple more, and turned ten of them in, which seemed to make him happy. He gave me a copy of the book, and a locket. The book is actually a fair read.
Anyway, taking down the "Keepers" meant we could get into the ruin. No sooner had we done so, though, then we were attacked by a dragon. We took it down with relative ease - with both Serana and I pelting it with spells, it had nary a chance - but it did summon Bonemen. No big problem, but I had expected to be able to consume its soul. Instead, I was bending over the puddle one of the Bonemen had left when I realized that the dragon wasn't burning - rather, it was disappearing.
Leaving that mystery for later, Serana and I accompanied Valerica and retrieved her scroll. She refused to come back with us, stating that it would put both the world and Serana in more danger. She told us that the prophecy that Harkon is seeking to fulfill is called "The Tyranny of the Sun", and if he uses Serana's blood to contaminate a weapon called "Auriel's Bow", he will be able to permanently darken the sun. As I already concluded, she also noted that vampires could not survive the crusade to remove the curse on the sun that would follow.
In addition, I'm not a fan of bleeding out Serana any more than Valerica is.
As we made our way out, we encountered the dragon again, significantly more friendly. He asked only that I summon him to Tamriel now and then, as he cannot otherwise leave the Soul Cairn. He also called me "Dovah", more evidence that I am dragon-born, I suppose. Perhaps I really should visit the Greybeards. No time to think of that now, though.
I have to say, it was good to be back in Tamriel, even in Valerica's ruined laboratory. I had worried that the Ideal Masters might try to trap us as they had trapped Valerica. For now, though, I've promised both Serana and Valerica that I'll return to tell Valerica once the coast is clear. And as Serana made clear to me when we left the Soul Cairn, if Harkon won't give up his desire to blot out the sun and kill his daughter, that means he has to go. Actually, Serana pointed out that Harkon will probably also want to kill me, as by accomplishing what he could not, I've proven to be stronger - and a threat.
We'll see about that.
In the meantime, we're off to find the last Elder Scroll. We've visited the Mages College again, who directed us to a hermit in the ice fields of the Sea of Ghosts. I made a bargain with the old man to bring him a copy of the Scroll in a Dwemer lexicon cube, which I shall. Eventually. At the moment, we need to find the scroll in this Dwemer ruin before anyone else does.