An Invader's Scourgestone is the insignia of the Scourge's elite troops. These are sometimes found on the stronger of the troops positioned in the Plaguelands. Slaying such a creature is indeed a challenge, but one that should be relished with a commission to vanquish evil.
The Argent Dawn currently offers a valor token in exchange for ten of these insignia. Bring them to me here, and I will make sure you receive your just reward.
Congratulations, <nom> - I am pleased to award you with an Argent Dawn valor token!
Each valor token handed out to worthy recipients is a sign of the tide turning against the Scourge. With more and more heroes fighting the Scourge in their own lands, the Argent Dawn will assuredly be at the forefront when the time comes to wage war on the ultimate evil of the Scourge. I hope that the valor you've shown what it took to acquiring your token will carry the day then as well!