I wrote this for my old guild awhile back, thought a few people on here might find it helpful.

To get your key to Onyxia, you must go to Morgan's Vigil in the Burning Steppes. That is where the chain starts.

1.) Quest name: Dragonkin Menace
Start: Helendis Riverhorn
Location: Burning Steppes
Level: 54 elite
Notes: The NPC asks you to kill a number of different types of dragons in Burning Steppes. I did this at level 60 with a level 60 warrior and myself. If you do this at level 54, I would advise taking more than two people. Some of the dragons have rather high HP and do a pretty decent amount of damage.

2.) Quest Name: The True Masters
Start: Helendis Riverhorn
Location: Burning Steppes
Level: 54
Notes: The NPC asks you to deliver a letter to Magistrate Solomon in Lakeshire.

3.) Quest Name: The True Masters
Start: Magistrate Soloman
Location: Lakeshire Town Hall
Level: 54
Notes: You are asked to deliver another letter to Highlord Bolvar Fordragon in Stormwind Keep.

(Darkflame add: The True Masters -- Speak with Lady Prestor.

4.) Quest Name: The True Masters
Start: Highlord Bolvar Fordragon
Location: Stormwind Keep
Level: 54
Notes: You are asked to take another letter back to Magistrate Solomon in the Lakeshire Town Hall.

5.) Quest Name: The True Masters
Start: Magistrate Solomon
Location: Lakeshire Town Hall
Level: 54
Notes: Magistrate Solomon tells you to return to Morgan's Vigil in the Burning Steppes and to talk to Marshal Maxwell.

6.) Quest Name: The True Masters
Start: Marshal Maxwell
Location: Burning Steppes
Level: 54
Notes: Marshal Maxwell tells you to find and speak to someone named Ragged John in the Burning Steppes. He is located almost directly North of Morgan's Vigil near a cave at the top of the map.

7.) Quest Name: Marshal Windsor
Start: Ragged John
Location: Cave in Burning Steppes
Level: 54 elite
Notes: This is where things start to get harder. You can start this at level 54, but I would not advise it until closer to level 60. Take a full group. You will be heading into Blackrock Depths for this. Marshal Windsor is located in one of the cells towards the right side of the entrance to the instance.

8.) Quest Name: Abandoned Hope
Start: Marshal Windsor
Location: Blackrock Depths
Level: 54 elite
Notes: Again, take a full group of five. The more level 60's you have the easier this will be. Once you talk to Marshal Windsor, he tells you to leave him and talk to Marshal Maxwell again. So it's back to the Burning Steppes for you.

9.) Quest Name: A Crumpled Up Note
Start: Crumpled Up Note
Location: Blackrock Depths
Level: 54 elite
Notes: After you talk to Marshal Maxwell, the chain seems to end. You must go back to Blackrock depths and kill enemies until one of them drops the crumpled note. It is a random drop, but luckily does not seem to take long to get. When you do find it, you must take it to Marshal Windsor, who is still sitting in his jail cell in Blackrock Depths.

10.) Quest Name: A Shred of Hope
Start: Marshal Windsor
Location: Blackrock Depths
Level: 58 elite
Notes: Marshal Windsor asks you to find his lost information. There are two pieces, they are dropped by General Angerforge and Golem Lord Argelmach.

11.) Quest Name: Jail Break!
Start: Marshal Windsor
Location: Blackrock Depths
Level: 58 elite
Notes: This is the infamous escort quest that you have probably heard about at some point in guild chat or in your travels. This quest is actually extremely easy to do, provided you take the right precautions. Before ANYONE accepts the Jail Break! quest from Marshal Windsor, you must clear the path that he will be walking to retrieve his gear. There are two main areas you must clear, the first being the ring in which his cell is located, the other being the Ring of the Law. Make sure you clear out the cells along the way as well. Marshal Windsor gives off an aura that inceases attack speed, not everyone in the party will get it, it is broken so don't worry about it. Even if you clear out the path, there will still be enemies that spawn to attack you. Make sure your tanks are on their toes, you don't want Marshal Windsor dying and he is a pretty weak guy at this point in time. And no you cannot heal him during the fights, he will refill by himself after each one is over. After you are done, go back to
Morgan's Vigil in the Burning Steppes and talk to Marshal Maxwell.

12.) Quest Name: Stormwind Rendezvous (Raid)
Start: Marshal Maxwell
Location: Burning Steppes
Level: 63 elite
Notes: Head to the gates of Stormwind. You may have to wait a few seconds for Marshal Windsor to spawn at the gates, once he does, talk to him to continue the quest chain.

13.) Quest Name: The Great Masquerade (raid)
Start: Marshal Windsor
Location: Stormwind gates
Level: 63 elite
Notes: You will now have to follow Marshal Windsor through Stormwind, talk to him to start the quest. He will stop to talk to General Marcus Johnathan, just let him yap and eventually you will be told to follow him to the keep. When you get to the gates of the keep, he will stop and tell you to give him the signal when you are ready. Just click him again and you will have to follow him into the keep. When you get to Highlord Bolvar Fordragon, you listen to some arguement and then Lady Onyxia will spawn. At this point, if you are melee get the hell out of the way and DO NOT try and help Bolvar. He will be fighting a bunch of level 62 elites. When I did it, a crit of 1300 damage didn't even budge one of their HP bars. Bolvar will be taking them down, you can do this with just one ranged class helping. Myself and a warrior did it after the buff the dragons got in the patch, I attacked whatever one Bolvar was, and the warrior just watched.

14.) Quest Name: The Dragons Eye
Start: Highlord Bolvar Fordragon
Location: Stormwind Keep
Level: 63 elite
Notes: Bolvar will give you a medallion, take it to Winterspring. You must find an NPC named Haleh. He is in a cave to the south/middle of the Winterspring map. The cave is full of elite drgaons and dragonkin. So either take a group, or get naked and start corpse running. At the back of the cave is a blue glowy thing on the ground. Step on it to get teleported to Haleh.

15.) Quest Name: Drakefire Amulet
Start: Haleh
Location: Winterspring
Level: 63 elite
Notes: Haleh tells you to retreive the Blood of the Black Dragon champion from General Drakkisath. This means you have to go to Upper Blackrock Spire. This will take a raid group of 15 people. The General drops anywhere from two to four blood per run now, so you won't have to deal with the nightmare of endless runs to get your one blood like I did. Once you do, return to Haleh and he will give you your key to Onyxia. Congratulations! Now you can join other players in attempting to kill her!

Notes: I listed the levels of each of these quests, but... I would STRONGLY recommend being level 60 before you even start this. I personally would be glad to help you with the first quest if you are only level 54. The rest of the chain you can do alone. Once it takes you into Blackrock Depths and Upper Blackrock Spire, don't even bother asking for help if you are not level 60 yet, you will probably get laughed at. Happy hunting to all and I hope this guide helped out those who needed it.