The S'ct'lac'tr is a virus bio-engineered by the S'pht to kill Pfhor in a matter of hours. The virus was released in the citadel as a last stand against the Pfhor, but it was ultimately not enough to win the war and the S'pht were enslaved. Later the Humans under command of Robert Blake used it in their base to destroy the Pfhor. But it was again uneffective as the Pfhor's cyborgs were immune to it, as were the Simulacrums. It also seems the virus didn't go away after a thousand years as the citadel was only infested by F'lickta and Pfhor cyborgs, who are immune to it.