Onyxia’s Lair is easily the most convoluted and time consuming process when it comes to Attunement. Both factions must complete a long quest chain, ranging all over Azeroth and deep inside the many dungeons of Blackrock Spire. The Horde and Alliance quest chains are actually quite different and so you’ll find them separated by faction below.
Each quest chain culminates in acquiring the Drakefire Amulet, which necessary to gain access to Onyxia’s Lair. Because of its ability to resist fire damage, the neck slot item will come in very handy during the fight with the Black Dragon and her progeny as well.
Accept the second quest: The True Masters. For this quest, just take the letter you’ve been given to Lakeshire in Redridge Mountains. Give it to Magistrate Solomon. Next you’ll need to make a stop in Stormwind, then talk to Ragged John in northern Burning Steppes. Head South to Marshal Maxwell once finished.
Accept the third quest: Marshal Windsor. For this quest, you’ll need to find Marshal Windsor. You’ll need to progress through Blackrock Depths to reach the prison area to find him. He’s in the cell on the most Southeastern part of the Detention Block.
Accept the fourth quest: Abandoned Hope. Simply head back to Marshal Maxwell at Morgan’s Vigil.
A Crumpled Up Note
To get the next quest, you’ll need to kill monsters in Blackrock Depths until you loot A Crumpled Up Note, which starts and completes the fifth quest: A Crumpled Up Note. Go back to Marshal Windsor in Blackrock Depths.
Accept the seventh quest: Jail Break! As the quest name suggests, you’ll have to assist Marshal Windsor in his escape from Blackrock Depths. Your best bet is to clear as many enemies as possible before starting the escort quest. Once successful, turn the quest in with Marshal Maxwell in Morgan’s Vigil.
Accept the eighth quest: Stormwind Rendezvous. This is a super simple quest, just go to Stormwind and speak to Squire Rowe. Afterwards, go to the entrance of Stormwind and speak with Marshal Windsor. He can be found outside the city gates of Stormwind, off of the road on some grass.
The Great Masquerade
Accept the ninth quest: The Great Masquerade. For this one, you’ll need to protect Reginald Windsor as he travels through Stormwind. Although it seems intimidating, Bolvar will actually do most of the fighting for you. Just focus on staying alive!
Accept the eleventh and final quest: Drakefire Amulet. Head to Upper Blackrock Spire. Clear through the dungeon until you kill General Drakkisath and loot his corpse. Return to Haleh in Mazthoril, Winterspring.
Once you get back, you’ll receive your fresh Drakefire Amulet, allowing you entrance into Onyxia’s Lair. Congratulations, attunement complete!
Head to Kargath in The Badlands to accept the first quest: Warlord’s Command from Warlord Goretooth. To complete this quest, you’ll need to kill Highlord Omokk, Overlord Wyrmthalak, and War Master Voone inside Lower Blackrock Spire. It might be wise to just clear the whole dungeon. Return to Warlord Goretooth in The Badlands upon completion.
Accept the second quest: Eitrigg’s Wisdom. This quest is short and simple. Go to Orgrimmar and talk to Eitrigg in Thrall’s Chamber. Speak to Thrall afterwards to finish up. He’s the one in the big chair.
Accept the fourth quest: What the Wind Carries. Another quick one here. Just listen to Thrall tell you his story.
The Champion of the Horde
Accept the fifth quest: The Champion of the Horde. For this one, you’ll need to locate Rexxar somewhere along the main road of Desolace. Your best bet is to start running from Shadowprey Village until you eventually run into him.
Accept the seventh quest: Oculus Illusions. You’ll need to collect a total of 20 Black Dragonspawn Eye from Upper Blackrock Spire. 2 of them drop from each dragonkin. Once you’ve collected all 20, head back to Myranda the Hag in Sorrow Hill.
For The Test of Skulls, Chronalis: Head to Tanaris and go to the Caverns of Time. Kill Chronalis, who is blocking the entrance and pick up his skull. This quest is turned in at Emberstrife in Dustwallow Marsh.
The Test of Skulls, Scryer
For The Test of Skulls, Scryer: You’ll need to find Scryer in Winterspring. Scryer is close to Mazthoril, hiding in a cave. You’ll know you’re in the right spot when you find the Haleh teleport. Don’t forget to loot his skull. This quest is turned in at Emberstrife in Dustwallow Marsh.
The Test of Skulls, Somnus
For The Test of Skulls, Somnus: Now you’ll need to go Swamp of Sorrows and head south of The Temple of Atal’Hakkar (Sunken Temple). Kill Somnus once you find him and loot his skull. he tends to travel all around south of the Temple, so you may have to search for some time.
The Test of Skulls, Axtroz
After killing Chronalis, Scryer, and Somnus, accept the twelfth quest: The Test of Skulls, Axtroz. To locate Axtroz, go to the Wetlands and head to the eastern part of the zone until you’re in Grim Batol. Kill and loot Axtroz for his skull. Return to Emberstrife in Dustwallow Marsh upon completion.
Upon your return, you’ll receive your shiny, new Drakefire Amulet, permitting you entry into Onyxia’s Lair. Congratulations, attunement complete!
That is the complete chain of quests for both factions, for attunement to Onyxia’s Lair. Whether Horde or Alliance, you may now enter the dungeon in Duskwallow and face Onyxia and the rest of her progeny. Good Luck!