You'll be sent to talk to Lord Harkon who will then lead you to the main hall for an announcement. After the announcement, talk to Serana and then make your way to the College of Winterhold. If you aren't already part of the College and haven't earned entry during the Main Quest, you will have to do one or the other to gain entry. Once inside the main building take the door leading to the Arcaneaum. Talk to the Urag gro-Shub there and he will direct you to Dragon's Bridge
When you get to Dragon's Bridge, you may be able to get the information you need from one of the guards but if you've progressed the civil war far enough, the guards may have been replaced by faction appropriate "Soldiers" who are unhelpful. You can still get the information you seek from a small boy in town. He usually strolls around town after 3:30pm or so. Talk to him and ask about a Moth Priest. He will direct you over the bridge.
Cross the bridge and keep travelling along the path until you come to an overturned carriage. Loot the designated body to find a note. This note will tell you that the Moth Priest has been taken to a nearby dungeon for protection. Head over to Forebears' Holdout (short distance by foot).
From the entrance, go down the ramp until you come to a large room with a stream running in front of you. Cross the stream and approach the structure. You'll be attacked by Dawnguard, wolves, and trolls. Most of them are melee enemies so you can try to draw them back out of the structure so that you have more backward space to move around with. Once you've kill them, head up the stairs in the structure to the spot where the giant shield is. Kill the Dawnguard up here and one of them will have a Waystone Focus. Take it, go upstairs and use it to lower the shield.
When you do so, the Moth Priest will attack you. "Kill" him and then you may have to wait. After a couple of days without feeding, you'll be granted the "Seduction" power. Use this on him and then try to talk to him. This will allow you to "feed" which will make him your Thrall. At this point, talk to him and direct him to the castle. Do so yourself and speak to Harkon. After that, talk to the Moth Priest to hear about the Elder Scroll.