The Bloodstone Chalice

Serving as the fifth quest in the add-on, represents the first quest after the fork in the storyline, where the Dragonborn chooses to side with Harkon and acquire the Vampire Lord ability.

Background
After accepting Harkon's offer, he asks the Dragonborn to speak Garan Marethi. Garan Marethi informs the Dragonborn of the Bloodstone Chalice, an ancient vampire artifact that needs to be filled with with water from the Redwater spring located deep within Redwater Den. He warns that the den has fallen into the hands of unsavory people who will also need to be dealt with.

Walkthrough
Head to Redwater Den located within the Rift. From the outside, it appears as a dilapidated hut guarded by a doorman who will welcome the Dragonborn.

Go around to the side of the hut and into a trap door that leads inside the den. Another doorman will be posted inside. Once inside, it becomes apparent that a secret skooma den is being run from within. Speaking with the dealer at the counter will give a little more insight into the den and some free Redwater Skooma samples can be obtained from her if the speech skill is high enough. Head past the cacophony of hacking coughs and towards the Adept level locked gate on the right. Pick the lock or pickpocket the key from the attendant and head into the cave system.

Skooma cave
Once inside, head through the tunnels and into a large cavern that seems to be where the Redwater Skooma is distilled. Fight past the hostile vampires and head up the ramp and around to the upper platform were a lever is located. Pull the lever and it will drop the wooden drawbridge. Cross the bridge and head deeper into the cave system killing any additional hostiles until the door to Redwater Spring is reached.

Redwater Spring
This part of the cave is actually an old Nordic tomb that was buried during an earthquake in the Second Era, and as such is infested with Draugr. Proceed through the tunnels and towards the main prison. Be careful to avoid a floor trap that leads to a spike pit when entering. Kill the vampires here and head forward. Venarus Vulpin, a powerful vampire and leader of the skooma operation can be found in the next set of rooms. Kill him and his thralls and head on until the spring itself is reached.

Walk up to the bubbling bloody waters and attempt to fill the chalice. At that moment Stalf ‎and Salonia Caelia, two vampires from Lord Volkihar's court will appear. They argue about who should get the credit for returning the chalice and then they decide to attack the Dragonborn.

Once they have both been killed, fill the Chalice with bloody springwater. Walk up to one of their bodies take a sample of their blood. Proceed through the tunnel and exit the cave system back into the skooma den. Return to Garan Marethi and tell him what happened.

An alternate method is to consume the Redwater Skooma while in the den. After its effects, the Dragonborn will wake up locked up inside a cell from within the dungeon. From here, the gate can be unlocked and the spring reached as normal.