After talking to Isran and Tolan, Isran tells you that he wants you to go see what the vampires were looking for in Dimhollow Crypt.

This starts the next quest in the Dawnguard series. Open your quest log:

I've met Isran, the leader of a group of vampire hunters known as the Dawnguard. The vampires that attacked the Hall of the Vigilants may have been after something in a place called Dimhollow Crypt. Isran has asked me to find what the vampires are looking for.

Find out what the vampires are seeking

Isran also tells you to feel free and poke around the fort and take what you need. There isn't much yet, but you're welcome to anything you can use. Vigilant Tolan tells you he will meet you at Dimhollow. It's the least he can do to avenge his fallen comrades.

Exit the Dawnguard Fort and back into the Dayspring Canyon. The Dimhollow crypt is to the north of Volunruud and to the east of Ustengrav, and also near the Lord Stone, so fast travel to one of those locations, then travel by foot, climbing up the mountain trail, until you have discovered the Dimohollow Crypt.

Enter Dimhollow Crypt. You hear a vampire talking to a vampire fledgling, saying how the Vigilants never know how to give up. The vigilant fought well though, Jeron and Bresoth were no match for him. The vampire says that all this talk is making him thirsty and hopes that another vigilant will wander in soon. The Vampire Fledgling is eager to return to the castle and report to Harkon, and the vampire threatens to tell Lokil of the fledgling's disloyalty.

You may barge in at any moment. Kill the vampires. You can loot them and also Vigilant Tolan who lays dead as well. There is a wooden gate here, but it is locked. Go up some stairs to find a chest with some loot. Activate the chain and go back down, and through the now open gate. Go through the tunnels, following the stream of water. You might encounter some skeletons here, and more Vampire Fledglings. This place is pretty dark, so if you have a torch, you should use it. Activate a lever to enter a chamber with some more chests in it. Go down the stairs through a narrow hallways and kill some Blooded Vampires, Death Hounds Restless Draugrs, and Draugr Wights, Draugr Deathlords, and Vampire Mistwalkers there.

You will have to activate another chain to open the next gate. Go down some more stairs and into another cavern. Arrive at a big chamber with a waterfall. Defeat the undead enemies there. A Volkihar Vampire will run to you and attack you, neutralize him as well.

Climb up to a platform at the opposite end of the room. Go through the hallways, killing additional enemies including more vampires, and frostbite spiders. Another lever opens the gate blocking your way, kill the Nightmaster Vampire there and go through the door on the other side. Keep exploring the caves, and find the door to Dimhollow Cavern.