Ruse of the Ashtongue

This is a quest involved in the Black Temple Attunement Chain:

The Black Temple attunement starts at The Altar of the Sha'tar for Aldors, and the Sanctum of the Sun for Scryers. Both are in Shadowmoon Valley.

Aldors start attunement from Anchorite Ceyla (http://www.wowhead.com/?npc=21402) at the Altar of Sha'tar in Shadowmoon Valley:
1. [A] http://www.wowhead.com/?quest=10568 [70] Tablets of Baa'ri
2. [A] http://www.wowhead.com/?quest=10571 [70] Oronu the Elder
3. [A] http://www.wowhead.com/?quest=10574 [70] The Ashtongue Corruptors
4. [A] http://www.wowhead.com/?quest=10575 [70] The Warden's Cage

Scyers start attunement from Arcanist Thelis (http://www.wowhead.com/?npc=21955) at the Sanctum of Stars:
1. [Sc] http://www.wowhead.com/?quest=10683 [70] Tablets of Baa'ri
2. [Sc] http://www.wowhead.com/?quest=10684 [70] Oronu the Elder
3. [Sc] http://www.wowhead.com/?quest=10685 [70] The Ashtongue Corruptors
4. [Sc] http://www.wowhead.com/?quest=10686 [70] The Warden's Cage

After that, the chain comes together for Aldor and Scryer.

5. http://www.wowhead.com/?quest=10622 [70] Proof of Allegiance
6. http://www.wowhead.com/?quest=10628 [70] Akama
7. http://www.wowhead.com/?quest=10705 [70] Seer Udalo

Seer Udalo is found in the Arcatraz (http://www.wowhead.com/?zone=3846) just before the last boss.

8. http://www.wowhead.com/?quest=10706 [70] A Mysterious Portent
9. http://www.wowhead.com/?quest=10707 [70] The Ata'mal Terrace
10. http://www.wowhead.com/?quest=10708 [70] Akama's Promise

After you finish this chain, you get to pick from some rewards and the chain seems like it ends. It continues however, inside Serpentshrine Cavern (http://www.wowhead.com/?zone=3607) from an NPC called Seer Olum (http://www.wowhead.com/?npc=22820) who you can speak with after killing Fathom-Lord Karathress. He starts the next part of the Black Temple Chain by offering the quest:

1. http://www.wowhead.com/?quest=10944 [70] The Secret Compromised
For this quest you just return to Akama.
2. http://www.wowhead.com/?quest=10946 [70] Ruse of the Ashtongue
In this quest, you must use the costume Akama provides, and kill A'lar in Tempest Keep: The Eye while wearing the Ashtongue Cowl. Attunement for The Eye is a whole different chain.
3. http://www.wowhead.com/?quest=10947 [70] An Artifact From the Past
In this, you need to gain access to The Battle of Mount Hyjal in Caverns of Time, and slay the first boss, Rage Winterchill. Attunement for this involves killing Lady Vashj and Kael'Thas Sunstrider.
4. http://www.wowhead.com/?quest=10948 [70] The Hostage Soul
For this you must speak with A'dal in Shattrath city.
5. http://www.wowhead.com/?quest=10949 [70] Entry into the Black Temple
For this, just speak with the naaru Xi'ri in front of Black Temple.
6. http://www.wowhead.com/?quest=10985 [70] A Distraction for Akama
Unknown what this quest asks of you, most likely you need to defend Xi'ri and make your way into Black Temple. At the end, you become attuned to enter and are granted The Medallion of Karabor (http://www.wowhead.com/?item=32649)

At this point, you are attuned and can enter Black Temple, although the chain continues and you can soon upgrade your medallion. It continues with:

1. http://www.wowhead.com/?quest=10958 [70] Seek Out the Ashtongue
You just need to find Seer Kanai (http://www.wowhead.com/?npc=23158) inside Black Temple.
2. http://www.wowhead.com/?quest=10957 [70] Redemption of the Ashtongue
This involves entering Black Temple, and killing the boss the Shade of Akama (http://www.wowhead.com/?npc=22841).
3. http://www.wowhead.com/?quest=10959 [70] The Fall of the Betrayer
After killing the Shade of Akama and returning to Seer Kanai, he asks one last thing of you. Kill Illidan The Betrayer. After doing so, you are granted the Blessed Medallion of Karabor, an upgraded version of your original medallion, and allows teleportation to the Black Temple.

Seer Olum says: Akama, my friend! Have you been informed? I was found out by Vashj's followers. Our secret is in peril!
Akama says: Olum... old friend. It is good to see that you are safe.
Seer Olum says: Illidan will find me and when he does... he will pry the Ashtongue secret from me - such is the power of the Betrayer! No... there is only one way. My usefulness to the cause has come to an end. I must venture into the spirit world!
Akama says: You cannot... be serious, Olum. Your sacrifice has already been too great.
Seer Olum says: My decision is final. I will not be dissuaded. I have devoted my life to our plan. To jeopardize it now would betray all who have died for the cause long before me.
Akama says: It appears your mind is set then. We will guide you to the spirit world with as much kindness as we can.
Seer Olum says: I thank you, Akama. You honor me by allowing me this moment, surrounded by my brothers. Farewell, friend. Guide our people well.
Akama says: Farewell, dear friend. We shall meet you in the next world when our duties in this one are fulfilled.
Akama says: Master! We've found the traitor who escaped Vashj! His body lies in front of me... lifeless!
Illidan's Presence says: You disappoint me, Akama. I wanted to question the treacherous worm myself! I'm beginning to question your allegiance, Broken.
Akama says: But, sire! You have misjudged my actions... I do have knowledge of someone close to you who plans to betray you. It is that dog, Kael'thas! He has allied with Kil'jaeden and intends to replace you as Lord of Outland!
Illidan's Presence says: I do not sense lies in your voice, Akama. That Kael'thas would betray me does not come as a big surprise - I'm not as oblivious as some would think. That one of your own was involved with him puts your loyalties into question.
Illidan's Presence says: Send your Ashtongue into Tempest Keep and slay his most prized possession, the phoenix known as Al'ar. I must know that you're not on his side. Do not think of betraying me, Broken. We both know who owns your soul!

Also-
If you are battle-rezzed you must re-equip the disguise or you will not get credit.