Harkon congratulates you for succeeding in finding the Moth Priest. Somehow he knew it would be you and Serana that would locate the Moth Priest. Tell him the Dawnguard tried to stop you but posed no thread. Harkon wishes he could have been there to hear the sound of their screams.

Your thrall awaits, and they have given him the Elder Scroll. Your objective is to command the Moth Priest to read the Elder Scroll and hear the words of prophecy.

Talk to the thrall, Dexion Evicus, who is standing near. Tell him you have a new task for him. Order him to read the Elder Scroll. He asks everyone to be quiet so he can concentrate on reading the Elder Scroll

The Reading of the Elder Scroll
I see a vision before me, an image of a great bow. I know this weapon! It is Auriel's Bow! Now a voice whispers, saying "Among the night's children, a dread lord will rise". In an age of strife, when dragons return to the realm of men, darkness will mingle with light and the night and day will be as one. The voice fades and the words begin to shimmer and distort. But wait, there is more here. The secret of the bow's power is written elsewhere. I think there is more to the prophecy, recorded in other scrolls. Yes, I see them now... One contains the ancient secrets of the dragons, and the other speaks of the potency of ancient blood. My vision darkens, and I see no more. To know the complete prophecy, we must have the other two scrolls.

Speak to Harkon. Ask him if he knows where the other scrolls are. HE tells you that his traitor wife stole one of them and then disappeared. As for the other, the last that he heard, it was lost in the bowels of a Dwemer ruin. It seems our work is not yet done. But I have waited this long, and we are so very close now. I can wait a bit longer.

This completes the quest, Prophet, and starts the next quest, Chasing Echoes.