Head to its other side, go up the stairs and pull the chain above the gargoyle.
Doing so will cause the statue to come to life and raise the grate nearby the stairs.
Behind it you will find a grate and a door beside each other. The road behind the wooden door is shorter, but requires lockpicking abilities. Both will lead you to a bridge above the room.
Head to the other side and keep going right. That way you will reach another gate, behind which you're in for a longer fight with gargoyles. After defeating the beasts, examine the armour lying nearby the coffins (it might come in handy) and afterwards pull the candlestick hanging on the left side of the fireplace.
New primary objective: Find Valerica's journal
That way you will unlock a secret passage which will lead you to the laboratory of the vampire you're looking for. Speak with your companion there and she will ask you to find her mother's journal.
The little red book can be found in the middle shelf of the candle-lit library. Take it to the vampire.
New primary objective: Search the lab for Soul Gem Shards, Purified Void Salts and Finely Ground Bone Meal
Your next mission will be finding three ingredients needed to create the portal.
The Soul Gem Shard can be found in the bowl standing on the table by the stairs.
The Purified Void Salts has been hidden on a shelf in the upper part of the room.
The Finely Ground Bone Meal lies in a silver bowl below a mammoth skull.
Having collected all of them, but them into the bowl where Serana is waiting for you. Soon afterwards a portal which can take you to Soul Cairn will be created.
In order to head through it, you need to fulfil one of two conditions. You either have to change into a vampire or let a part of your soul to be taken from you. Both rituals can be performed by Serana. The disadvantage of the first solution is that you won't be able to complete the Dawnguard side missions (no problems with the main story line), while the second option will take away a part of your statistics. You however will be able to recover them after you obtain the scroll.