Use the Empty Termite Jar on the Termite Mounds in Eastern Plaguelands. After you've gathered 100 Plagueland Termites, return to Nathaniel Dumah at Chillwind Camp in Western Plaguelands.
I've tried many times to rout out the Scarlet Crusade through force alone, but to no avail. So, as an alternative, I've decided to make their lives more difficult.
If you are willing to help, I want to infest that lumber mill with the most terrible species of termites ever found.
In Plaguewood, near the once-great Stratholme, there is a breed that survived the plague and the Scourge combined. Find their mounds and fill this jar with them, so we can prepare them to be unleashed on the Crusade.