Onyxia Attunement: How To Do
The Alliance Quest Chain

This quest chain was removed from the game with patch 3.0, since Onyxia no longer requires any attunement. This was what you once had to do as Alliance to get the Drakefire Amulet and enter her lair.

1. Go to Morgan’s Vigil in Burning Steppes (it’s where the Alliance FP is) and accept "Dragonkin Menace" from Helendis Riverhorn. You must be 48+ to get it. It was soloable as a 55 hunter but I decided to have a level 60 hunter help me for speed. The dragons are on the far east side of Burning Steppes.
2. Go back to Helendis Riverhorn and turn in "Dragonkin Menace", accept "The True Masters pt. 1"
3. You need to deliver a letter to Magistrate Solomon in Redridge Mountains. Turn it in and accept "The True Masters pt. 2"
4. Now deliver this letter to Highlord Bolvar Fordragon in Stormwind City. Accept "The True Masters pt. 3".
5. Speak with her (she’s right next to him), then turn it back in to Bolvar. Accept "The True Masters pt. 4".
6. Deliver the letter to Magistrate Solomon in Redridge Mountains. Accept "The True Masters pt. 5"
7. Go back to Burning Steppes to Marshal Maxwell and turn it in. Accept "The True Masters pt. 6".
8. Go and speak to Ragged John, then go back to Marshal Maxwell and turn it in. Accept "Marshal Windsor"
9. Go to Blackrock Depths, kill High Interrogator Gerstahn for the Prison Cell Key, go back a bit to Marshal Windsor and speak to him. Turn in "Marshal Windsor" and accept "Abandoned Hope"
10. You need to go back to Marshal Maxwell in the Burning Steppes and turn in "Abandoned Hope". He won’t give you a follow-up.
11. Instead you have to go back to BRD and kill mobs until A Crumpled Up Note drops. It can also drop from the mobs outside BRD in Blackrock Mountain. I had it drop from an Anvilrage Warden (elite) so you can solo this outside the instance if you want to. Right click on the note and accept the quest A Crumpled Up Note. Turn it in at Marshal Windsor in the cell in BRD. Accept "A Shred of Hope" from him.
12. You now have to kill Golem Lord Argelmach and General Angerforge inside BRD, loot Marshal Windsor’s Lost Information from both of them, and return to Marshal Windsor. I soloed it as a level 70 hunter no problem, but you’ll need a group of 57-60s preferably if you’re not 70. Deliver the 2 pieces of lost information to Marshal Windsor in his cell again.
13. DON’T accept "Jail Break!" until you clear the ring his cell is in (not the part where High Interrogator Gerstahn is though), the open area you see straight ahead as you enter the instance, and the Halls of the Law which is the ring that goes around outside the arena. THEN you can accept the "Jail Break!" escort quest. Don’t fail or you will have to reset the instance to reaccept it as far as I know.
14. Once you’re done, go back to Morgan’s Vigil in the Burning Steppes and turn it in to Marshal Maxwell. Accept "Stormwind Rendezvous" and fly to Stormwind.
15. Go out to the front gate and talk to Squire Rowe. He will summon Windsor. Turn in "Stormwind Rendezvous" and accept "The Great Masquerade". Just follow him, stand back and let Bolvar do the fighting. Easily soloable, just don’t take aggro. Turn it in at Bolvar and accept "The Dragon’s Eye"
16. You need to go to Haleh in Winterspring. You can run through the cave just east of where she stands and in the back is a blue circle which teleports you to her. Bring a group or be prepared for multiple corpse runs if you do want to go through the cave. Turn in "The Dragon’s Eye" and accept "Drakefire Amulet".
17. This last step is the same for Horde and Alliance in that you have to go to UBRS and kill General Drakkisath and loot his blood. A raid of about 10 level 60s is a good idea for this part. Bring the blood to Haleh to complete "Drakefire Amulet" and get your amulet.

As for Onyxia’s Lair, it’s located in Dustwallow Marsh at 53.77. You need to wear the Drakefire Amulet or have it in your backpack, be in a raid and be at least level 50 to enter.