Dragonkin Menace -> The True Masters (part 1, Redridge) -> The True Masters (part 2, SW) -> The True Masters (part 3, back to Redridge) -> The True Masters (part 4, back to Morgan's Vigil in Burning Steppes) -> talk to Ragged John -> talk to marshal Maxwell -> Marshall Windsor -> Abandoned Hope. Next step begins when you find a Crumpled Note off random drop in BRD. Take it back to Marshall Windsor and get A Shred of Hope -> Jail Break -> Stormwind Rendevous -> The Great Masquerade, and you then get the key to Onyxia's Lair!

To quote someone that wrote it up so nicely at the END of the quest listings, I thought I would move it up to the front, so that anyone printing out the steps, could save some paper.

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By revoluti0n, Alliance Drakefire Amulet quest chain

To enter Onyxia's Lair, you must endure these steps:

1) Dragonkin Menace (Elite) from Halrendis in Morgan's Vigil, Burning Steppes.
2) Talk to Marshal Maxwell in Morgan's Vigil.
3) Talk to Magistrate Solomon in Lakeshire.
4) Talk to Highlord Bolvar Fordragon in Stormwind.
5) Talk to Magistrate Solomon in Lakeshire.
6) Talk to Marshal Maxwell in Morgan's Vigil.
7) Find Marshal Windsor in Blackrock Depths jail.
8) Return to Maxwell.
9) Find a random "A Crumpled Up Note" drop from a random dwarf in Blackrock Depths.
10) Take said note to Marshal Windsor.
11) Retrieve Marshal Windsor's Lost Information from General Angerforge and Golem Lord Argelmach (two pieces).
12) Escort Marshal Windsor out of Blackrock Depths jail.
13) Meet Marshal Windsor at gates of Stormwind.
14) Escort Marshal Windsor to Stormwind Keep to unveil who Lady Katrana Prestor really is. (Spoiler: Onyxia)
15) Stand way the hell back while Bolvar fights the elite dragons.
16) Travel to Winterspring to find Haleh. Find lots of elite blue dragons and run to the back of the cave, where you'll see a rune on the floor. It's hard to see, so you will die a few times.
17) Haleh tells you to go kill Drakkisath in Upper Blackrock Spire. You will need 15 fairly skilled players for this quest, including yourself.
18) Once you have the "Blood of the Black Dragon Champion", return to Haleh. Drakkisath drops only ONE of these per kill. Yes, one for the entire 15 man group. Not one per person, one per group.
19) Return to Haleh with the Blood and claim your Drakefire Amulet. My 59 paladin now has his. :)

After you turn this quest at Marshal Maxwell, he doesn't give you a follow-up quest. Instead you have to return to Blackrock Depths and kill mobs until you find A Crumpled Up Note, which continues the chain.