The two women went down the portal leaving Beor behind since he couldn't enter the Soul Cairn without draining his soul. He also didn't want to become a vampire or be soul trapped so he stayed behind, securing that no one would enter it.
Leandra went down the staircase watching that she wouldn't trip. Serana? What do you know about the Soul Cairn? I mean what things are living here or who is in charge of this place? She asked.
Look at this place. Who would like to live here? The only things which are "alive" if you could call it a life, are trapped souls and skeletons. The lords of this place are the Ideal Masters. No one knows who they are exactly or how they look like. The only thing we know is that they want souls. No matter what the cost.

They went on to search Valerica.
They often do exchanges with necromancers but they trick them and lure them here... and they can't escape. Serana said and looked around the dry place.
I hope we can return to Tamriel. Leandra said worried. I don't plan to live here for the rest of my life.
Serana chuckled. Don't worry, we'll get out. As long as the portal is opened. Otherwise it will be difficult to do. She said. Do you smell this? It's disgusting. Come on we should hurry.

As they went on they stumbled uopn skeletons with black bones, wich rose from the ground. Leandra finished one with a lightning spell and Serana one with an ice spike.
Damn that's creepy! Leandra exclaimed as the skeletons got fluid.

After a while they came across a large fort with some kind of barrier in front of it.
That looks like a hiding place. Maybe your mother is inside of it. Leandra said to Serana who smiled at those words.

They went nearer and saw a woman with the same armor as Serana's.
Serana? Is that truly you? What are you doing here? I must have failed. The woman said shocked and went from one side to the other.
Mother! We're looking for the Elder Scroll you had. We want to stop father but we need it to pervent him from getting it. Serana told Valerica.
Are you insane?! You can't stop him! No one can! He's far too powerful. And now you've brought another pure vampire to him. He now has two options of completing the prophecy. You fools how could you not- Valerica screamed as she heared Serana.
Do you really think he's more powerful than the Dragonborn? Leandra asked her and interrupted her screaming.
No. Why are you asking? The Dragonborn doesn't exist and who are you? Valerica asked annoyed.
I am Leandra the Dragonborn if you please. Honored to meet you. She bowed sarcasticly before Valerica.
Oh. I-I see. Valerica said. Forgive me. I didn't know. If you want the Elder Scroll it will be yours. But as you can see I am a prisoner but you can lift this barrier. There are three keepers and everyone guards one point of energy of this barrier. Kill them and return. But be aware of an ancient dragon, his name is Durnehviir.
And so Serana and Leandra searched the keepers to kill them.

They found the first, an archer and after a furious fight they were able to kill it.

And so they found the second, wich was wielding a battleaxe made of dragonbones.

After a while the third. (Of wich I dindn't make a screenshot )

They walked back to Valerica's castle.
You surpise me. I didn't think you'd made it. Follow me the Elder Scrolls are all yours now. She said and opened the door to the courtyard.

As they walked over it they could hear the sound of wings flapping.
It's Durnehviir! We need to kill him! Valerica shouted.
Leandr drew her sword and ran towards the decaying dragon. She could hear Serana's ice spike spell and an unkown sound of Valerica. But before the three women could even come close the dragon summoned skeletons and flew away.

After a while he landed and Leandra used that cance to hit him with her sword. She could hear a skeleton behind her but she knew its weapon would bounce of her armor.

After an enduring fight the dragon finally fell but Leandra didn't devour his soul.
That dragon... it was decaying. How can it even conntinue to exist? Serana asked her mother.
Valerica smiled. That, Serana, is the Soul Cairn. Here is nothing logical. Come now. I have your Elder Scroll.

Leandra took the Elder Scroll.
Valerica? Why don't you come with us? She asked.
I'd love to but then Harkon would have another option to complete this damned prophecy. You know what? If you're able to kill him come to me and I will come home. Valerica said, tears in her eyes.
Leandra walked away to leave Serana and Valerica some time together.

She opened the door and saw Durnehviir manifestating in front of her. She wanted to unsheath her blade as he began to spoke.

Stay your weapons. I'd wanted to speak with you, personally. You slayed me and no one did when I still roamed Tamriel. Could you do me a favor and call my name in Tamriel. I want to smell the fresh air and see the trees and mountains. If you do this I'll grant you my Thu'um. He said and awaite Leandra's answer.
Of course. It would be my honor. She said.
Thank you. With this words he flew away.

As soon as they went out of the portal, Beor ran towards them.
Leandra! Serana! I've been so worried about you! How was it in this Soul Cairn? He asked happy about their return.
Leandra felt a warmth in her heart as he held her in her arms. It's not a place where I'd want to live. Everything there is dead. She said to him.
By the divines! Do you have the Elder Scroll? He wanted to know.
Yes we do. Let's search the third.