Even vampires have bogeymen.
Geneviève Lacroix wasn't really made to be a leader. Perhaps that's why she chose to form a council instead of an empire; she wouldn't have to bear the burden of responsibility alone. While the Council of the Undead was a success, her inability to face challenges head-on would affect vampires all over the world.
When Coventry Payne informed Geneviève of her intent to betray her sire, Grant Black, Gené did nothing. When Coventry succeeded, Gené ignored her own laws and Coventry went unpunished. When Coventry showed up asking for her own House, Gené gave her New York. When Coventry began weaving myths of Grant's excessive violence and ruthlessness to keep fledglings in line, Gené remained silent. When Geneviève feared Coventry would make a play for the Paris Council House, she gutted it instead of fortifying it.
As Coventry continued to grow her power and influence, Grant lay bound in a box, alone in the dark, starved of blood, slowly going mad. Only Coventry hadn't counted on the ever-increasing population of humans, the "shrinking" of the world, the eventuality that someone would stumble upon his prison and release him.
One hundred and fifty years has seen that eventuality come to pass, and now he will have his revenge. Coventry will get the bogeyman she created — although the monster she claimed he was is nothing compared to the monster he's become.