Lysinda was ancient vampire and the leader of the circle of vampires named after her. She and her circle were powerful and respected among other vampires.
The time of Lysinda's becoming a vampire is not known; she was already undead during the War of the Bug Squishers, when she lurked in Lord Torgamous's castle, masquerading as a gypsy seer, with the intention of converting Lady Valerie into one of her vampires. While there, she killed Advisor Magon Coifer and destroyed K'Z'K's horde of deadels sent to kill everyone in the castle, because both would have interfered with her plans for Valerie. She eventually offered her the choice to become a vampire after her husband the warlord died, and she accepted.
In modern times, Lysinda and her circle resided in a mansion in Maine, trying to recover from an attack from Muffin the Vampire Baker. Lysinda personally oversaw the creation of new vampires to replace ones lost; as part of the conversion ritual, they had to drink her blood. But as Torg and Zoë were among the potential converts, the vampires found themselves under attack by Riff, Aylee, Kiki and Bun-bun (he was looking for the TV remote). While many vampires were killed, Lysinda proved to be too tough for the humans, who were all knocked down at the end of the encounter - except for Zoë. Lysinda offered her the choice of becoming a vampire again, but she refused. Before Lysinda could finish her off, Bun-bun distracted her, whereupon Zoë took one of the stakes lying around and stuck it through her heart from behind. Thus incapacitated, Lysinda was taken out to the yard along with all of her circle except Sam to be turned to dust at sunrise.
After her destruction, Lysinda has become Satan's girlfriend in Hell, although it's not known whether they're still together.