Enter one of the booths and consume the Skooma. Your vision will become blurry. You lose consciousness. You wake up locked up. Listen to the conversation between the vampire and the dealer. The Skooma was a trap, meant to lure innocent people and make them thralls. The water from the spring makes the Skooma more potent.

You can try talking to the vampire from behind the bars, but he tells you that he does not converse with his prey.

There are three ways to get out of the cell:

Lockpicking the door
The cell gate has an adept level lock. If you have lockpicks, you can try picking the lock

Transforming into a Vampire Lord
If you transform into a Vampire Lord, you will grab the attention of the Vampire standing outside. He will open the door and enter the cell to attack you, and you can kill him and escape.

Even if you do not have lock picks and you do not want to turn into the Vampire Lord just yet, there is another way of braking out of the cell. The corpse of a dead novice conjurer lays nearby. Near him, you can find: a spell tome, telekinesis. Telekinesis lets you pull an object from a distance. Learn it and find it under Alteration magic. Use it to pull the Redwater Den Backrooms Key from the table close to the cell. When it drops to the floor, you can just pick it up. Use the key to open the door.

No matter which way you choose to break out, the vampires outside will attack you. Now would be a good time to change into a Vampire Lord if you haven't already. Leave the cell. Kill the enemies outside, including the vampire, vampire thralls, the dealer, and the attendant. There are also some Death Hounds around.

Explore the huge complex of underground tunnels. There is a lot of stuff to loot here. Eventually you reach a wooden door that requires you to activate a chain to open it. Then open an Iron Door. The next hallway is fool of traps, either go around the marked tiles, or hover over them as a Vampire Lord. You reach another iron door. Go through it and enter a big chamber. Kill the skeletons and Vanarus Vulpin inside. There are also Master Vampires, Blooded Vampires and more Vampire Thralls inside.

Make sure to search the body of Venarus Vulpin. He is carrying a Redwater Wellspring Key. Use it to unlock the Iron Door that has a Master Lock. Enter the wellspring area. You now need to fill the chalice from the spring. Activate the Bloodspring. You have to return to Garen Marethi. But first, you need to defeat Stalf and Salonia. Salonia Caelia and Stalf shouldn't be hard to kill. Then Fill the Chalice with the Vampire Blood from Stalf.

Exit Redwater Den and fast travel to Castle Volkihar. Talk to Garan Marethi. This completes the quest, The Bloodstone Chalice, and starts the next quest, Prophet.