This quest chain is for the Alliance only. I dont think the horde gets this follower. Unsure about her horde equivalent.
Edit: for clarification and apparently by popular demand lol.
I don't get any kind of notification when a response is posted to a comment (if there is a way to, I don't know it). So I just discovered a bunch of people upset about some info in the post. Sorry about that >.>
Check out all the quests here before starting. A lot of them are picked up at the same time. They arent necessarily in any particular order with a few exceptions. BASICALLY do all the quests after you get Speaker for the Dead.
This quest chain, which leads to gaining the follower [npc=79426] begins with Speaker for the Dead not to be confused with Speaker for the Dead
Then
Holding the Line
Then Powering the Defenses
Then
We Must Construct Additional Pylons
and Nightmare in the Tomb
The Heart of Auchindoun
Disrupting the Flow
Optional - Every Bit Counts
Light's Rest
Antivenin
note: Into the Hollow does come before Scheduled Pickup. As of 11/22/14 it is showing them in reverse order in the quest chain quick facts box.
Into the Hollow
Vile Defilers
Scheduled Pickup
Restalaan, Captain of the Guard
Optional - Never Forget
Optional - Payback
Optional - Invasion of the Soul Eaters
Then The Final Piece not to be confused with The Final Piece
Changing the Tide
Desperate Measures
Optional - Ogre Diplomacy
Into the Heart of Madness
Optional - Retribution for the Light
Optional - Hiding in the Shadows
Optional - Book Burning
Leading to this quest. You must complete the chain to open Destination: Unknown for the follower.