Update: You no longer need a Drakefire Amulet to enter Onyxia's Lair! I've left the guide up so old-timers can show their friends what they had to go through to fight Onyxia.
The dragon Onyxia is the boss of a 40-man end-game raid in World of Warcraft. To enter her lair everyone in the raid needs a Drakefire Amulet, often called an "Onyxia key." Obtaining a Drakefire Amulet involves completing one of the longest quest chains in the game. It differs depending on whether your character is Alliance or Horde. These are the steps required for Horde players.
1. Warlord's Command (LBRS)
At level 55 or higher you can get the "Warlord's Command" quest from Warlord Goretooth. He's at the top of a tower in Kargath in the Badlands and has no exclamation mark over his head. You will have to slay Highlord Omokk, War Master Voone, and Overlord Wyrmthalak with a 5-man group in Lower Blackrock Spire. You also have to recover some important documents which spawn randomly at one of the 3 bosses. Since version 1.7, everyone on the quest can loot the documents.
2. Eitrigg's Wisdom
Upon completion of "Warlord's Command," Goretooth will ask you to speak with Eitrigg in Thrall's chamber, which is in the Valley of Wisdom in Orgrimmar. Eitrigg wants you to talk to Thrall, who gives you the next quest.
3. For the Horde (UBRS)
Here you should have a 10-man raid to kill Rend Blackhand in Upper Blackrock Spire. Rend is a little over halfway through the instance, and most raids going to UBRS want to go at least that far. Bring Rend's head back to Thrall in Orgrimmar.
4. What the Wind Carries
Thrall has some things to tell you. All you need to do is have a nice chat with him in his chamber to complete this quest.
5. The Champion of the Horde
Now you will be sent to find Rexxar, who travels through Desolace from the Stonetalon border to the Twin Colossals in Feralas, then turns around and heads back.
6. The Testament of Rexxar
Rexxar wants you to talk to Miranda the Hag in the Western Plaguelands. You'll find her near Uther's Tomb, just south of the Ruins of Andorhal.
7. Oculus Illusions (UBRS)
Miranda sends you back to Upper Blackrock Spire to collect 20 Black Dragonspawn Eyes. These drop for raid groups and are quite plentiful. You shouldn't have any problem getting enough unless almost everyone in your raid is on the quest, in which case you can always kill some Dragonspawn in the Rookery.
After taking the Dragonspawn Eyes to Miranda, she will give you an Amulet of Draconic Subversion and tell you to speak with the dragon Emberstrife, who is in a cave on the southern perimeter of Dustwallow Marsh. When you use the Amulet, it will disguise you as a Black Scalebane so that you can talk to Emberstrife without being attacked. Make sure you get the first 3 "Test of Skulls" quests before moving on.
9. Test of Skulls, Somnus
Each of these quests requires you to kill an elite dragon and take its head back to Emberstrife. A 5-man group is best for these quests, but with a good healer they can be done with less. It's not always easy to find willing people to help out because there is a lot of travel involved. Somnus can be found on the east side of Swamp of Sorrows.
10. Test of Skulls, Chronalis
Chronalis is an elite dragon in Tanaris near the mountain on the east edge of the zone. You'll probably have to kill a couple lower-level drakes to get to him.
11. Test of Skulls, Scryer
Scryer is in the back of the caves of Mazthoril southwest of Everlook in Winterspring. It's well-populated with elite dragonkin so be prepared to fight your way in. Once you have the heads of Somnus, Chronalis, and Scryer take them back to Emberstrife in Dustwallow Marsh.
12. Test of Skulls, Axtroz
When you reach Emberstrife you'll have to go back into dragon form using the Amulet to turn in the heads. Emberstrife will then give you one more elite dragon to kill. This time it's Axtroz, who is patrolling a canyon in Grim Batol on the east end of the Wetlands.
Once you've taken Axtroz's head to Emberstrife in Dustwallow Marsh, you'll be sent back to Rexxar in Desolace, who will give you the last quest in the chain.
14. Blood of the Black Dragon Champion (UBRS)
Rexxar needs the blood of General Drakkisath from Upper Blackrock Spire to restore the Dull Drakefire Amulet. Drakkisath is the last boss in the instance, and you'll need a 10-man raid to take him down. Although you can't loot the blood without the quest, he only drops 2-4 blood each time, so depending on how many people in your group are on the quest, you may have to roll for it. When you take the blood back to Rexxar he will give you your Drakefire Amulet, which grants you access to Onyxia's Lair. Congratulations!