From the frontpage of the module:
Thanks to the quick thinking of many adventurers, Greyhawk was warned of the impeding dooom of Safeton months ago. The Greyhawk Militia sent some 200 men to the walled city in hopes to stop this supposed new slavelord and it looks like it may have. Nothing unusual has happened since the troops arrived. Could it be the slavelord decided it not to contend with the might of Greyhawk or was he simply biding his time? A one-round Core adventure in the "Slaver" series. It is recommended that you have played COR5-12* Calm Before the Storm and COR7-05 City of Malice before playing this adventure.
The correct number should read COR6-12.
Further adventures that are prequels to the series are COR5-05 A marked Man and COR5-12 Return to the Undercity.