Report to Lieutenant Doren at the Rebel Camp in Northern Stranglethorn.
Several years ago, Stormwind's Captain Kurzen brought a brigade into Stranglethorn Vale on a mission whose details are still murky. Kurzen swiftly descended into madness, as did his men... except for a few rebels who made it their mission to undo the evils that Kurzen had wrought.
As I said, that was years ago. I'm now hearing rumors that Kurzen is dead, but those rebels still persist at their rebellion. If you wish to investigate, you'll find them at a camp on Stranglethorn's northern border.