Baron Sablemane has told you that in order to make the poison that Rexxar needs, you must acquire 7 Crust Burster Venom Glands. Return to him at the Circle of Blood in the Blade's Edge Mountains once you have done so.
So, your Rexxar would have me fashion a poison for him? I'll give him what he desires, but there will be a price. Even for friends, there must be a price. Free things destroy friendships.
First, you must venture a short distance to the southwest to the Churning Gulch.
Therein mill some of the most poisonous creatures in all of Outland, or anywhere. I speak of the crust bursters, and this poison will require gathering their glands for me.