You need to take the Bloodstone Chalice to the Redwater Den and fill it in the Redwater Spring. Exit the castle. If it is daylight, it will say "Your vampire blood boils in the sunlight". This is because of an active effect, Weakness to Sunlight, which causes Health, Magicka and Stamina to not recover while in Sunlight and reduces them by 15 points.

Check your quest log:

The Bloodstone Chalice
I've been asked to take a special artifact of Harkon's - the Bloodstone Chalice - and to fill it from the spring in Redwater Den. After that, I need to add the blood of a powerful vampire and deliver the chalice to Garan Marethi

Objectives
Take the Bloodstone Chalice to Redwater Spring.

Redwater Den is close to Riften, so fast travel to the town. You can buy a horse in the Riften stables if you want to travel faster. Travel the road west towards the den, past Merryfair Farm and through a forested area. Soon you reach Redwater Den which looks like a wooden shack. A Redwater Lookout tells you to get lost if you are not here for business. There is a female lookout sitting inside the hut. On the room next to where she is, near a barrel, there is a door on the floor. Open it and enter Redwater Den.

There are two ways to do the quest. One way, is to just kill everybody, open the back door to the Redwater Den Backrooms, and locate the Redwater Spring yourself. But this walkthrough will show you the more interesting way to do this quest.

Go down some stairs. The doorman says that if you are going downstairs, keep the weapon to yourself. Go further down. You hear a Bandit Outlaw and a Nobleman coughing. Talk to the dealer inside the cage. She tells you that you can buy from her, then join any booth you'd like.

Your dialog options are:
What is this place?
What have you got for sale?
How about a sample? (Persuade)
What is so special about your Skooma?

If you ask her what is this place, she tells you that it is the Redwater Den, purveyors of the best Skooma outside of Balmora. If you ask her what is so special about their Skooma, she tells you that the secret is their mystic Redwater Spring. The magic in the waters opens the mind to experience Skooma in a way like never before.

You can try persuading her to give you a free sample. If your persuasion succeeds, she agrees, seeing it is your first time. She tells you to find a booth and enjoy.

You receive:

Redwater Skooma
Restore 40 Stamina

It's time to look for a booth and get high by having some Skooma.