It seems in the next step of the Legendary questline, after capturing Garona Khadgar has a plan to use her to track down Gul'dan to finally dispatch of Gul'dan. Yet during this event Gul'dan, will escape and in the process take control of Garona in which you will free her by gaining control of the Orb of Dominion and will then hopefully destroy it.
It should maybe play out like this:
*You, Khadgar, Cordana Felsong, and Garona arrive at a Shadow Council lair.*
Garona: This is a highly fortified high-security bastion. Only a fool would go in there without an army. I mean... good luck, Champion.
Cordana Felsong: I don't like this.
*You and your party enter the Shadow Council lair.*
*You come upon a chamber with the Orb of Dominion inside.*
Khadgar: Come with me. This is exactly what I need to undo Gul'dan's hold on Garona.
Cordana Felsong. That is dark magic you're working with.
Khadgar: We're using the tools of our enemy against them.
Cordana Felsong: You are using the tools of our enemy!
*Garona most likely stabs you.
Insert you name here: [Player name] gasps in pain.
*Garona then most likely attacks.*
Garona: Haven't you given up already, mage?
Khadgar: Not 'til my dying breath. I will end your torment, Garona.
Cordana Felsong: Quickly, destroy that thing!
*Khadgar begins an incantation.*
Khadgar: Om na ka om, Fel na om kal...
*A battle between you and Garona ensues. Cordana will most likely aid you. You are already defeated in battle until Khadgar completes his incantation and Garona is free from Gul'dan's mind control.*
Khadgar: Awaken, Garona. You are free at last.
Garona: Gul'dan... he's not in my head anymore... I owe everything to you! To both of you!
Khadgar: Come with me, Garona. Together we will plan for Gul'dan's downfall.
Garona: I will do whatever it takes to stop Gul'dan. Wherever you lead, I will follow... commander!
*The quest ends and you are awarded with Garona, Master Assassin.*