Onyxia

Hello folks its been a while. RL has taken its toll on me these last few weeks and I haven't felt like writing much. Unless you want to hear of my family troubles.... lol
So, to that point and with the most recent patch I'm going to hit some lore. While there is plenty out there to find I'd like to take a shot at the wondrous Black Dragonflight.
From what I can tell Onyxia first appeared in patch 1.12 or very close to that. She began as a forty-man raid for level 60. With the new patch 3.2.2, she has been updated to bring 10 and 25 man raiding back to the Dragonmurk area of Dustwallow Marsh.
The attunement for her used to be long and arduous for us Alliance folks. Beginning in BRD with the release of Marshall Windsor it takes you back and forth to Winterspring before you finally reveal Onyxia's true form in Stormwind. There is no longer an attunement for Ony, however the quest chain is still available.
Onyxia is the daughter of Neltharion (aka Deathwing) and his one remaining consort Sintharia (aka Lady Sinestra).
Ony thrives on creating dissension and political upheaval in the mortal races around her, and took on the guise of Lady Katrana Prestor, an advisor to Anduin Wrynn, while the King was being held captive. Her brother is Nefarion, the infamous last boss in Black Wing Lair. It is said that she worked with the Defias Brotherhood in orchestrating the capture and detainment of Varian Wrynn so that she could press her interests in Stormwind and the Alliance. She would use her political clout to pit the various factions in the area against each other, and to keep them away from herself and her brother Nefarion.
Neltharion is promised to make an appearance in the upcoming expansion, Cataclysm, as the reason why all the upheaval in the world of Azeroth takes place. I can't wait to see it actually. Lady Sinestra can be found now in Outlands in Shadowmoon Valley where she makes a brief appearance to the orcs on the Netherwing Ledge asking for more Netherdragon eggs to be delivered to her. According to the lore laid out in the book "Night of the Dragon" she resided in Grim Batol for a time, bent on creating a new dragon race to take over the world.

If you are interested in doing the quest chain:
Alliance begin with Marshal Maxwell at the Morgan's Vigil fight point in Burning Steppes, the quest is "Marshal Windsor" and it is to find Marshal Windsor in BRD (Black Rock Depths). At one point after he is rescued the chain dies and you need to find the quest "A Crumpled Note". It is a drop from mobs in BRD, usually around his jail cell area.
Horde begin with Goretooth who is located in a tower in Kargath in the badlands. That quest chain is called "Warlord's Command". Having not done it I can't give specifics about the chain.
Doing a search in WoWHead for either of those quest chains will get you started.

For further info on Onyxia lore any of the main wow sites can be accessed. Here are a few links:
WoW Wiki
WoWHead
There are also a few books out there in the Warcraft series that deal specifically with the Black Dragons. "The Demon Soul" from the trilogy "War of the Ancients" covers Deathwings rise and fall as well as the first encounters with the Burning Legion, the beginnings of Alliance, druidism and of course Queen Azshara and the "Sundering". "Night of the Dragon" covers Lady Sinestra and her plots. Both of these books also have the stories of Korialstratz (Krasus; the red dragon consort to Lady Alexstrasza) and Rhonin and Vareesa, the two figures now residing in the Violet Citadel in Dalaran.

For me, the Black Dragonflight family has always been very interesting. Perhaps because of the evil and secrecy surrounding them.