Tell Lord Harkon that you are ready to carry out his task. He tells you to go and speak to Garan Marethi.

This will complete the quest, Bloodline, and start the next quest:

The Bloodstone Chalice
Speak with Garan Marethi

Before leaving this chamber, you can practice your powers on the peasant (Vampire Cattle) standing near the door. Try and use a power that will increase your vampire experience. You will then see on the screen the message "Life Blood absorbed, vampire perk progress increased".

Exit the room and find Garan Marethi. If you cannot talk to him, revert back to your normal form first. Try looking for the Revert Form power in your favorites menu, if you cannot find it. Tell Garan that you have a message from Harkon. Marethi tells you about the Bloodstone Chalice. It is a rather well-kept secret. The chalice has been in Lord Harkon's possession for quite some time now. It is, when used properly, able to increase the potency of our powers. He has neglected using it for ages. If he wants it filled now, then grander things are afoot. This is good. Lord Harkon has never cared to use the Chalice, relying on his more than adequate powers until now. This is an intriguing change.

Follow Garan into the dining room. He announces that he needs to fetch the Bloodstone Chalice. Vingalmo and Orthjolf are shocked when they hear this, but Garan tells them it is Lord Harkon's orders. Vingalmo and Orthjolf are Harkon's primary advisors. They are surprised to learn that you are taking the chalice.

Garan presents you with the Chalice. This chalice needs to be filled directly from the bloodspring that is the source of Redwater Den. But that's not enough. Once that's done, the blood of a powerful vampire needs to be added to it. He tells you to keep in mind that Redwater Den has fallen into the hands of less reputable members of the society who now dwell there, and that it is your choice whether to deal with the residents of Redwater Den, or force your way through. He tells you to hurry back, you don't want to keep Lord Harkon waiting.

The Bloodstone Chalice is added to your inventory. Your objective now is to take the Bloodstone Chalice to Redwater Spring.