The Mayfair Legacy is an important facet of the Lives of the Mayfair Witches. On several occasions, the Mayfair Legacy is discussed in Blood Canticle (2003), bridging the Mayfairs into the Vampire Chronicles.
When the spirit of Lasher establishes his plan to become incarnate through selectively inbreeding thirteen generations of the Mayfair witch family, he also provides the Mayfairs with a vast fortune. By the time of the sixth witch, Marie Claudette Mayfair, the Mayfair family is incredibly wealthy. When she establishes the family in Louisiana, she also establishes the Mayfair Legacy to ensure that one witch is the Designée in control of the family wealth, but on two conditions: First, she must keep the family name Mayfair on the day of her marriage and, second, she must wear the Mayfair emerald—a beautiful necklace with a large emerald. The Designée and wearer of the Mayfair emerald is a female witch (except for Julien Mayfair) and has the power to control Lasher. Rowan is the last Designée of the Mayfair Legacy to control the evil spirit. The next Designée, Mona, does not need to control Lasher, since by then he is already dead.
For more perspectives on the Mayfair Legacy in the Vampire Chronicles, read the Alphabettery entries Designée of the Mayfair Legacy, Julien Mayfair, Lasher, Mona Mayfair, and Rowan Mayfair.