You'll have to travel to the Ancestor Glade just outside of Falkreath. The dungeon is quite simple actually. Go up the left side and across the large tree that forms a bridge. On the other side, go down through the small cave until you come out in the glade. Make your way down to the center and you'll find the Knife that you need for the ritual.
Go to any of the trees with the colorful leaves here and use the knife on it. Now just make your way around the room and walk through the groups of moths. You should be able to find all seven groups without leaving this room. When you have them all, make sure you are ready for battle because once you are done reading the scrolls, you will be attacked by the vampires.
With all seven swarms, head back to the center of the glade and walk into the column of light. Stand in it and read the Elder Scroll (Blood). After you've read all three scrolls, talk to Serana.