The rite of blood and bone was a ancient rite practiced on Korriban in the Sith Academy.
To complete the rite, an acolyte was to enter a secret room and claim a skull from a bone pile protected by shyracks, an altar to the Academy's failed apprentices. Once the skull had been collected, the acolyte had to bathe it in a pool of blood. This would summon an arcane creature called the Bloodfiend, and the initiate had to defeat the beast to prove their worth.
Throughout history, many Dark Lords perpetrated the ritual. At the time of the Cold War, Overseer Ragate was the keeper and administrator of this old rite. Both the heir of Aloysius Kallig and the Empire's Wrath might have successfully taken the rite under Ragate's administration.