Reclamation of Gilneas

Reclamation of Gilneas is granted after finishing Reclamation of Gilneas Questline in Patch 10.2.5.
You'll get The Wall Between Us or What We Left Behind depending on your faction, just after completing Beginning a New Dawn.
Besides, you'll be rewarded with [item=211873] mount, [spell=430283] transmog, and [item=211788] toy.
It doesn't matter race or faction, log out then come back online after finishing the entire questline. You'll have the toy in your Toy Box, but is usable for Worgens only.

If you are on The Wall Between Us, Lilian and Queen Calia in Pyrewood Village are having a conversation.

Calia Menethil says: Thank you for your assistance, Funkowah. I know I asked much of you in aiding the Gilneans.
Calia Menethil says: I only hope that Tess continues to prove more... diplomatic than her father.
Lilian Voss says: I know her well. She cares far more about what people do than what they are.
Lilian Voss says: Still, it's probably best if we give the Gilneans some space for the time being. Let them enjoy their homecoming.
Calia Menethil says: As they allowed the Forsaken to do, not too long ago. We've kept our word. It's time we returned home as well.

If you are on What We Left Behind, Greymane asks for some time, as he needs to visit somewhere, and Tess asks us to check on him in What We Left Behind. Turns out, Greymane is visiting Liam Greymane's tomb to talk to his son and his regrets on building the wall.

Genn Greymane says: I don't know if you would recognize me, Liam.
Genn Greymane says: All these years, the lessons I have learned. The Alliance has always offered an outstretched hand.
Genn Greymane says: There are days it feels right, and days I wonder... what if I had not built that wall?
Genn Greymane says: Would Lordaeron still stand? Would you still be...
Genn Greymane says: Ah, but listen to me. The ruminations of an old man.
Genn Greymane says: You did not live long enough to regret as I do.
Genn Greymane says: Your sister will lead the kingdom, now.
Genn Greymane says: No one could be more proud of her than your mother and I. Except perhaps you.
Genn Greymane says: Rest well, my son. Gilneas is in the best of hands.

Okay, I just finished the questline that this achievement is connected to, and wow did Blizzard screw this up.

I'm not really even talking about the story, but if Tess runs Gilneas as well as she helps the player through this whole quest-chain then it was better off abandoned.

These have to be some of the worst designed quests. If you thought having to slog through elites in Zaralek Caverns with Ebyssian and Sabellion with you was bad, then are you in for a shock.
This is those quests but instead of staying by your side and focus firing on your target, Tess, (your companion NPC), is Shadow-Stepping everywhere and pulling elite mob after elite mob. All the while failing to pull any aggro off of the player. Sound fun?

Just an absolute failure of quest design. It's like the decided to force the player to use that Lillian Voss extra attack, but gave it too long of a cooldown making it not that effective against the legion of elites that the "brilliant" Tess has pulled.

Haven't been this disappointed in a quest since the Hati reunion quests.

Horde is supposed to start the quest with Clandestine Movements but the starter NPC is reported to be missing for some players so may have to start with To Gilneas now.

Alliance start with Summons to Lord Greymane