It sounds like you have earned Thorg'izog's trust, and he's assigned you to discipline duty. This is fortunate.
We need you to take out his sergeants. You'll find them on wolfback, outside the Blackrock Stronghold in the Fields of Honor. Use that cudgel on them, several times, until they die. You'll have to act fast, because if they realize you're not one of them, they can unmask you. Strike quickly and ferociously, <name>.
Completion
Without those sergeants, Thorg'izog will be hard-pressed getting his troops ready for battle.
Already the dominoes are beginning to fall, <name>.
Notes
Before leaving Thorg'izog's postion after accepting I Am the Law and I Am the Lash, Troteman radios in with this quest. Accept it before getting started. The cudgel hits for a quarter of the sergeant's health as well. So unload.
Is that a mask you're wearing, soldier?
What do you think you're doing, soldier!?
I'll slice you in two! Wait a minute... who are you?
You've been spotted! Club your target to death before you are unmasked!
The quest completes automatically after the fifth sergeant is killed.