The Sinstar Virus was an alleged mutation of the Monstar Virus that had manifested inside the children of Coheed Kilgannon and Cambria Kilgannon. It was said to carry the same destructive power as Monstar, only it was far more unstable. Supposedly it could be triggered randomly and could not be cured or contained. With no way to cure it, Coheed and Cambria were told they needed to murder their children so that the Keywork would be spared destruction.
In reality, no such virus ever existed. It was a ploy fabricated by Mayo Deftinwolf and Wilhelm Ryan to ensure the demise of the Crowing before they could ever appear. While they were successful in killing Josephine Kilgannon, who held the cure to the actual virus, as well as infants Matthew and Maria Kilgannon, a series of unforeseen events allowed the plot to be discovered and Claudio Kilgannon to be spared. This would prove to be a grave misfortune for the Supreme Tri-Mage as Claudio was indeed the Crowing and would seek to destroy Ryan in the years that followed.