This is part of a huge quest chain, but it tells epic stories about the game. It is amazing, and probably the best line we've ever participated in! Each side has their own chain, so I've listed all the quests you'll need to complete for each. The Alliance chain starts with The Lost Courier. He's located just southwest of Death's Stand. The Horde chain starts with Into the Fold. Located at (14,49) west of Azjol Nerub.

Alliance

1. The Lost Courier - Speak to Courier Lanson at Star's Rest.
2. Of Traitors and Treason - Show the Alliance Missive to Palena Silvercloud at Stars' Rest.
3. High Commander Halford Wyrmbane - Deliver the Alliance Missive to High Commander Halford Wyrmbane at Wintergarde Keep in eastern Dragonblight.
4. Naxxramas and the Fall of Wintergarde - Speak with Gryphon Commander Urik at the gryphon station in Wintergarde Keep.
5. Flight of the Wintergarde Defender - Rescue 10 Helpless Wintergarde Villagers and return to Gryphon Commander Urik at Wintergarde Keep. Use the provided Wintergarde Gryphon Whistle to rescue the villagers. They are located in the Carrion Fields South of Wintergarde Keep. You don't need to target them to pick them up, but you will need to stand still! The gryphon also has a "soar" option that should be used as often as possible. After you pick up the villager just return them to the gryphon master. You can only pick up one at a time.
6. Return to the High Commander - Speak with High Commander Halford Wyrmbane at Wintergarde Keep in Dragonblight.
7. Rescue from Town Square - High Commander Halford Wyrmbane at Wintergarde Keep in Dragonblight wants you to rescue 6 Trapped Wintergarde Villagers. The villagers are inside the buildings just outside Wintergarde Keep. All you need to do is kill the undeads near them to rescue them.
8. Find Durkon! - Seek out Cavalier Durkon at the Wintergarde Crypt in Dragonblight. Durkon is a dwarf at 79,53, right at the entrace to the crypt.
9. The Noble's Crypt - Cavalier Durkon at Wintergarde Keep wants you to slay Necrolord Amarion. He's inside the crypt, but be aware there will be a curse put on you that spawns undeads every few seconds when you pick up the tome. They have low HP, but if you go AFK before turning in the quest, you may come back to find yourself dead!
10. Secrets of the Scourge - Deliver the Flesh-bound Tome to Cavalier Durkon at Wintergarde Keep in Dragonblight. This is the book you picked up!
11. Mystery of the Tome - Take the Flesh-bound Tome to High Commander Halford Wyrmbane at Wintergarde Keep in Dragonblight.
12. Understanding the Language of Death - Take the Flesh-bound Tome to Inquisitor Hallard at the Wintergarde Keep prison.
13. A Righteous Sermon - Wait for Inquisitor Hallard to finish his Righteous Sermon and return to High Commander Halford Wyrmbane at Wintergarde Keep in Dragonblight with the information you uncover. Please note that this quest must be completed individually. Meaning, if you are in a group, each one of you has to sit throught the sermon. It's a few minutes long, so be patient!
14. The Demo-gnome - Speak with Siege Engineer Quarterflash at Wintergarde Keep in Dragonblight.
15. The Search for Slinkin - Find Slinkin the Demo-gnome in Wintergarde Mine. Use Quarterflash's Homing Bot if you need help finding the mine. The entrance to the mine is 80,45
16. Leave Nothing to Chance - Retrieve a Wintergarde Mine Bomb and use it to blow up the Upper Wintergarde Mine Shaft and the Lower Wintergarde Mine Shaft. Report back to Siege Engineer Quarterflash at Wintergarde Keep in Dragonblight when you have completed this task. The bombs are collected inside the mine. Use them at 80,44 and 80,45 (the spots will sparkle).
17. The Bleeding Ore - Siege Engineer Quarterflash at Wintergarde Keep in Dragonblight wants you to recover 10 samples of Strange Ore from Wintergarde Mine.
18. Understanding the Scourge War Machine - Deliver Quarterflash's Package to High Commander Halford Wyrmbane at Wintergarde Keep.
19. Into Hostile Territory - Speak with Gryphon Commander Urik to secure a ride to Thorson's Post. Report to Duke August Foehammer when you arrive at Thorson's Post in Dragonblight.
20. Steamtank Surprise - Use an Alliance Steam Tank to destroy 6 Plague Wagons and deliver the 7th Legion Elites to Wintergarde Mausoleum. Speak with Ambo Cash inside the Wintergarde Mausoleum in Dragonblight should you succeed. The steam tanks are around the carrion fields. Hop in one and destroy the plague wagons. The wagon has a few abilities, including one that freezes all the mobs around you. In order to destroy the wagon, you'll need to drop off the gnome. Once you've destroyed the wagons, head over to the Mausoleum at 85.50 and drop off the soldiers.
21. Scattered To The Wind - Ambo Cash at the Wintergarde Mausoleum in Dragonblight wants you to recover 8 Wintergarde Munitions. The Wintergarde Munitions are scattered throughout the Carrion Fields and not inside the Mausoleum.
22. The Chain Gun And You - Ambo Cash at the Wintergarde Mausoleum in Dragonblight wants you to save 8 Injured 7th Legion Soldiers. Use the nearby guns to kill the mobs that are attacking the soldiers. On your pet bar, there should be an ability to call out a soldier. You don't need to target the mobs to attack them as the gun will have a radius on the floor. Just make sure they make it safely to the other side. It's usually best to do this with a buddy.
23. Plunderbeard Must Be Found! - Ambo Cash at the Wintergarde Mausoleum in Dragonblight wants you to find Plunderbeard. His location is 84,54.
24. Plunderbeard's Journal - Recover Page 4 of Plunderbeard's Journal, Page 5 of Plunderbeard's Journal, Page 6 of Plunderbeard's Journal, and Page 7 of Plunderbeard's Journal and return them to Ambo Cash at the Wintergarde Mausoleum in Dragonblight. The mobs that these drop off of are actually down a small tunnel beside Plunderbeard. It's easy to miss as it looks like a dead end.
25. Chasing Icestorm: The 7th Legion Front - Report to Legion Commander Tyralion at the 7th Legion Front in central Dragonblight. The coords for the 7th Legion Front are 64,27.
26. Chasing Icestorm: Thel'zan's Phylactery - Recover Thel'zan's Phylactery from Icestorm and return it to High Commander Halford Wyrmbane at Wintergarde Keep. When you're ready to fight, go speak with "Wyrmbait." This guy is a pretty hefty elite, so let the NPCs take most of the beating. After you've killed him, the Phylactery will be a lootable item on the ground, not off the dragon itself.
27. Finality - Take Thel'zan's Phylactery to Legion Commander Yorik inside the Wintergarde Mausoleum in Dragonblight. 82,51 is his location.
28. An End And A Beginning - Defeat Thel'zan the Duskbringer and report to High Commander Halford Wyrmbane at Wintergarde Keep in Dragonblight. There is a scene before the fight that leaves you cowering in fear. After you are done being stunned, you will fight Thel'zan. Once again, you may want to let the NPCs take the beating if possible.
29. To Fordragon Hold! - Venture to Fordragon Hold in Dragonblight and speak with Highlord Bolvar Fordragon. Forgdragon Hold is at 42,29.
30. Audience With The Dragon Queen - Seek out Alexstrasza the Life-Binder at Wyrmrest Temple in Dragonblight. There's a flight path at Fordragon Hold, so fly out to Wrymrest Temple if you have the path for there. If not the Temple is at 60,53.
31. Galakrond and the Scourge - Speak with Torastrasza at Wyrmrest Temple in Dragonblight.
32. On Ruby Wings - Destroy 30 Wastes Scavengers and recover the Scythe of Antiok. Return to Alexstrasza the Life-Binder at Wyrmrest Temple should you complete this task. The Waste Scavengers are around 54,34. The nearby hill is where Thiassi is. You must kill him first to spawn Grand Necrolord Antiok. Dismount and kill him then loot the scythe from the floor.
33. Return To Angrathar - Speak with Highlord Bolvar Fordragon at Fordragon Hold and tell him of your victory over the Scourge. This will complete the achievement for you! We highly reccomend that you continue the chain to complete The Battle for Undercity as it's one of the most fun quests we've encounterd!

Horde

1. Into the Fold - Emissary Brighthoof at the Westwind Refugee Camp in the Dragonblight wants you to recover 10 Horde Armaments.
2. Blood Oath of the Horde - Speak to the taunka at Westwind Refugee Camp and make 5 of them pledge their allegiance to the Horde.
3. Agmar's Hammer - Travel to Agmar's Hammer in the Dragonblight and speak to Overlord Agmar. 37,45 are the coords for Agmar's Hammer.
4. Victory Nears... - Speak to Senior Sergeant Juktok at Agmar's Hammer.
5. From the Depths of Azjol-Nerub - Explore the Pit of Narjun and return with your findings to Senior Sergeant Juktok at Agmar's Hammer. To complete the quest just run near the entrance to Azjol Nerub (26,50). Remember that you must jump down into the hole to reach it!
6. The Might of the Horde - Senior Sergeant Juktok at Agmar's Hammer in the Dragonblight wants you to use the Warsong Battle Standard at Icemist Village and defend it against attackers. The best spot to use the standard is probably around 25,40. Just protect it from oncoming mobs.
7. Attack by Air! - Speak with Valnok Windrager at Agmar's Hammer.
8. Blightbeasts be Damned! - Use Valnok's Flare Gun to call down a Kor'kron War Rider at Icemist Village. Mount the Kor'kron War Rider and then use it to kill 25 Anub'ar Blightbeasts!
9. Strength of Icemist - Find Banthok Icemist at Icemist Village. The village is at 22,41.
10. Chains of the Anub'ar - Banthok Icemist at Icemist Village in the Dragonblight has asked that you bring him Anok'ra's Key Fragment, Tivax's Key Fragment and Sinok's Key Fragment. Anok'ra is at (24,43), Sinok is at (25,44), and Tivax is at (23,39).
11. Return of the High Chief - Free High Chief Icemist using the Anub'ar Prison Key and assist him in defeating Under-king Anub'et'kan. The chief is at 25,40.
12. All Hail Roanauk! - Seek out Roanauk Icemist at Agmar's Hammer and initiate him as a member and leader of the Horde forces. He's at 36,45.
13. Imbeciles Abound! - Travel to Venomspite in Dragonblight and speak with Chief Plaguebringer Middleton. His location is 77,62
14. The Forsaken Blight and You: How Not to Die - Chief Plaguebringer Middleton at Venomspite in Dragonblight wants you to bring him 10 Ectoplasmic Residue. Slay the Forgotten Riflemen at 83,66
15. Emerald Dragon Tears - Chief Plaguebringer Middleton at Venomspite in Dragonblight wants you to collect 8 Emerald Dragon Tears. You can find these around 63,70 on the ground.
16. Spread the Good Word - Chief Plaguebringer Middleton at Venomspite in Dragonblight wants you to use the Forsaken Blight Spreader's Blight Bomb to destroy 30 Hungering Dead on the outskirts of the Carrion Fields.
17. The Forsaken Blight - Deliver The Forsaken Blight to Doctor Sintar Malefious at Agmar's Hammer. 36,48 is the location!
18. The Kor'kron Vanguard! - Report to Saurfang the Younger at the Kor'kron Vanguard. Kor'kron Vanguard is at 40,18.
19. Audience With The Dragon Queen - Seek out Alexstrasza the Life-Binder at Wyrmrest Temple in Dragonblight.
20. Galakrond and the Scourge - Speak with Torastrasza at Wyrmrest Temple in Dragonblight.
21. On Ruby Wings - Destroy 30 Wastes Scavengers and recover the Scythe of Antiok. Return to Alexstrasza the Life-Binder at Wyrmrest Temple should you complete this task. The Waste Scavengers are around 54,34. The nearby hill is where Thiassi is. You must kill him first to spawn Grand Necrolord Antiok. Dismount and kill him then loot the scythe from the floor.
22. Return To Angrathar - Speak with Saurfang the Younger at the Kor'kron Vangaurd and tell him of your victory over the Scourge. This will complete the achievement for you! We highly reccomend that you continue the chain to complete The Battle for Undercity as it's one of the most fun quests we've encounterd!