Phase 4 of the reclaiming of Quel'Danas is Discovering Your Roots. This quest begins at the newly unlocked Alchemy Vendor, Mar'nah. Of course, she has been unlocked, but not her stock. She claims to need a special reagents for that: Razorthorn Root. She says it's fairly difficult to harvest, but this quest shouldn't give anybody too much trouble.
After accepting this quest, fly out to Razorthorn Rise. This region is directly on the border between Terokkar Forest and Hellfire Peninsula, up in the mountains. If you cant find it, just look at the picture above.
This is similar to the quest I Was A Lot Of Things... from Shadowmoon Valley, in that you need to use some local beasts to dig up your plants for you. Luckily, you don't need to rely on the intelligence of some dumb pig that gets massacred by all of the nearby monsters and turns on you if you accidentally hit an AOE. You get to control the Ravager you'll use to dig up your five required Razorthorn Roots.