Return to Eastern Plaguelands and track down the Crimson Courier. Kill the Courier and recover the Grand Crusader's Command. Take the command to Nathanos Blightcaller for further instruction.
Just as the body cannot survive without the head, the head cannot survive without the body. The defenses of the Scarlet Crusade's command inside Stratholme are almost impenetrable. We must, instead, cut the body out from under the head.
My Deathstalkers have been collecting data on the activity of the Crusade outside of Stratholme. Each day, a report is sent from their central command to Tyr's Hand. This report is the key, [race]! Find the Crimson Courier and recover that report. Return it to Nathanos.