Luxien

"'Voidwalkers are boring. I am trying to get… Something more exciting out of it.'"The mad scientist of the Burning Legion whose love for experiments is beyond description in words. Her fascination with the Void led her to creating the Dark Contract, her very own force of ultimate destruction

Biography
Nothing much is known about Luxien's past. She had been born in the Twisting Nether as a daughter of two Dreadlords and has a brother, who later started a rebellion and ran away. Meanwhile Luxien ascended thorough ranks in the Burning Legion and was doing various inventions and experiments for her master, Demonlord Archimonde.

It is mentioned she used to experiment with the undeath but seeing to future in it, she began working with the Void itself, coming as far as contacting several Voidlords, imprisoning and subduing them and later draining their power with which she infused herself and her minions, thus creating the Dark Contract.

Partly losing her sanity, partly never having it, she decided to track down her brother Taedal and bring him home. She arrived at Azeroth, the unfortunate landing of her ships causing the Forgotten Isle drift halfway to the Twisting Nether. After finding out Taedal is not going to return willingly, she set her base at the Forgotten Isle to bring her brother back to his senses by force.

In the Voidforge
The base of Luxien's operations on Azeroth is called the Voidforge and it is the heart of the Burning Legion's experiment with the Void. Serving as the home to the Dark Contract's leadership and prisoners, this place is extremely dangerous.

Luxien within the Voidforge serves as the raid end-boss and is the ultimate and final enemy in the Broken and Distant Worlds expansion.

Siblings forever
She clearly cares about her brother, despite being obsessed with controlling him. Taedal and she, despite standing on opposite sides, have an ongoing correspondence. Taedal had expressed regrets he hadn't taken Luxien with him away and that he has to fight her in the Voidforge, hinting that he will aid the Heroes in the final battle personally.

Trivia

 * Luxien's name is delivered from Latin word lux, meaning light. It could also be her connection to the Void, as lux stands for vacuum cleaner in Czech.
 * While her green fel tattoos have straight lines with sharp corners and are symmetrical, her blue void tattoos are assymetrical in the form of curvy spirals. This is to emphasize the contrast of the Void and Legion combined within her.
 * Luxien tells Taedal, that he was right about her experiments with undeath having to future, indicating that Void isn't the first field she is experimenting with, as well as Taedal being interested in her work.
 * Although Taedal was probably just subtly trying to get information out of her, his panic attack after he learned Luxien works with the Void tells us that Luxien wasn't toying with it when the rebellion began.
 * Since Luxien is often seen infusing powers of whatever she is working with into her minions and prisoners alike (and often in herself), it is entirely possible she had done it with the undeath too, which would explain several Undead Demons and Death Knights under her command.
 * Given she served under Archimonde and being a mad scientific genius, she could had probably stood at the creation of the Lich King.
 * If Taedal and his fate is a parallel to Illidan, it puts Luxien in Malfurion's role. That surprisingly fits as both of them have no control whatsoever over their brother, who caused a lot of trouble home and then left with plenty of followers to fight the Legion, both absorbing aspects of what they work with (Luxien's Void tattoo and tendency to phase away, Malfurion's "look") and both being the family voice of reason.
 * Despite being a Nathrezim, her features greatly resemble a Draenei or a Ma'nari Eredar, leaving the possibility she isn't entirely pureblood Dreadlord but s crossbreed instead. Since Taedal bears similar resemblance, it could run in the family.
 * Or it could be because her creator, when drawing her, still had the terms Nathrezim and Ma'nari confused and messed it up, which is far more probable.