After defeating the Boneyard Keepers, your objective is to speak to Valerica again. Go back to where the barrier was, and find out you succeeded in bringing down the barrier. You ask Valerica if she is able to give you the Elder Scroll now. She instructs you to follow her. She suggests that you keep watch for Durnehviir. With the prison's barrier down, he is almost certain to investigate.

Check your quest log:

Beyond Death
Serana and I have slain all three of the keepers and the barrier around Valerica's prison has faded. We rejoined Valerica, who has agreed to take us to the location where the Elder Scroll is hidden.

Follow Valerica

Valerica opens a door behind her and goes inside. Enter after her, into the Boneyard. The dragon, Durnehviir, shows up. Slay Durnehviir. Then, speak to Valerica and follow her again. Retrieve the Elder Scroll (Blood) from the case she opened. Talk to Valerica once again. You can ask her to come back to Tamriel with you, but she refuses.

You now have in your possession the Blood Elder Scroll, which is what you came here for. It is time to leave the Soul Cairn and return to Tamriel.

Exit the Boneyard. You come across the Dragon Durnehviir again, who has rematerialized. He wants to talk to you. He refers to you as the "Qahnoarin", or "Vanquisher", because you managed to defeat him in battle. Tell the dragon you found him equally worthy. He is honored to hear that. He wants to ask a favor of you. He has roamed the Soul Cairn for countless years, in unintended service to the Ideal Masters. Before this, he roamed the skies above Tamriel. He wishes to return there. But he is afraid that if he leaves the Soul Cairn, he will be weakened.

Ask how you can help. Durnehviir tells you that he will place his name with you and grant you the right to call his name from Tamriel. If you do him this simple honor, he will fight along your side as your Grah-Zeymahzin, your ally. and teach you his Thu'um. He tells you to speak his name to the heavens when you feel the time is right.

This gives you the following objectives:
Summon Durnehviir in Tamriel
Learn the first word of Durnehviir's shout.

It should now say:
Word of Power learned - Curse, Summon Durnehviir.
Word of Power learned - Never, Summon Durnehviir.

Follow the quest marker on the compass to where you entered Soul Cairn. Climb the stairs, and take the portal back to Volkihar Ruins.

This completes the quest, Beyond Death.