The Bloodstone Chalice (quest)

Walkthrough
After Bloodline and having accepted Lord Harkon's gift, turning the Dragonborn into a vampire, things now will turn up a gear.

Speak with Garan Marethi
Speak to Garan Marethi at Castle Volkihar and deliver Lord Harkon's message, though vague, Garan understands.

Follow Garan
Garan leads the Dragonborn to the chamber, off the main hall, where the Bloodstone Chalice resides on a pedestal. Pick it up.

Take the Bloodstone Chalice to Redwater Spring
Travel to Redwater Den, and enter via floor hatch. The skooma dealers there are less than thrilled with the Dragonborn's presence, especially since the back room area is off limits. Fighting the way through the vampires and their thralls, finally leads of the Redwater Spring.

Fill the Chalice from the spring
About to fill the chalice with the spring liquid...

Defeat Stalf and Salonia
... Stalf and Salonia show up. The former is the underling of Orthjolf and the latter of Vingalmo, and the arrivals are both trying to win favour with their masters. Their masters on the other hand are playing a political game, trying to become vampire lord themself... sadly Lord Harkon's new favourite, the Dragonborn, is not part of their plans.

So Stalf and Salonia need to be killed.

Fill the Chalice with vampire blood
After that rude interruption, fill the chalice with the vampire blood from the spring.

Return to Garan Marethi
Exit the den, return to Garan Marethi and give him the filled chalice, that from now on will have special powers. He also explains the vampire politics going on at Lord Harkon's court, and tells the Dragonborn to talk to the Lord Harkon.