Deliver Baron Sablemane's Poison to Rexxar at Thunderlord Stronghold in the Blade's Edge Mountains.
Here, take this to your master. I'm sure that whatever the reason, Rexxar will put it to good use.
Oh, and do be certain to remind him that he owes me.
He knows what I want, and I'm certain that however he intends to obtain it for me, I'll be seeing you again sooner than you think.
Now, do be a good little $g boy : girl; and run this up to Rexxar.