1. Go to the Badlands and talk to Warlord Goretooth

2. Do the Quest Warlords Command http://www.thottbot.com/?qu=4903

3. Talk to Eitrigg http://www.thottbot.com/?qu=4941

4. Kill Rend Blackhand in UBRS http://www.thottbot.com/?qu=4974

5. Turn in the head and go talk to Rexxar after receiving the quest http://www.thottbot.com/?qu=6567

6. Talk to Myranda the hag after getting http://www.thottbot.com/?qu=6568 you will then receive http://www.thottbot.com/?qu=6569

7. Get the black dragon eyes

8. Return to Myranda

9. Find Emberstrife http://www.thottbot.com/?qu=6570

10. Kill all three of the Dragons for the Test of Skulls
http://www.thottbot.com/?qu=6582
http://www.thottbot.com/?qu=6583
http://www.thottbot.com/?qu=6584

11. talk to Emberstrife for the final Dragon killing quest
http://www.thottbot.com/?qu=6585

12. Talk to Rexxar for http://www.thottbot.com/?qu=6602

13. Kill General Drakkisath

14. Talk to Rexxar to receive your Drakefire Amulet AKA the Onyxia Key

A useful thing to do is to complete the Blackwing Lair Key quest while doing the last part of the Onyxia key line
Read (our website link for BWL key) for more Info.