Magnataur live in prides, much like lions: one or two dominant males and several females along with their offspring. In the case of Gammoth, the magnataur den west of here, Gammothra is the leader. It is Gammothra that controls this magnataur population and therefore it is Gammothra that must be slain to prevent any type of magnataur uprising.
The powder can be used to weaken all magnataur. It is why the rat men of the area guard it so closely.
A job well done, <name>! Would you honor Laurith and I by taking the head of the beast and presenting it to Hellscream? He will be extremely pleased.