Disclaimer: I don't own Harry Potter or anything therein. No profit was made from this etc. etc. etc.
Drove Her To
Prison had been kinder to both Lucius and Bellatrix than most. They had escaped, she having done so before his sentence had even began and he breaking free within a year and a half. It helped to have the Dark Lord on your side, and it was that thought which carried them through their terms in Azkaban. Being reunited with him meant everything. However, being reunited with each other meant almost as much.
Bellatrix and Lucius had been carrying on a quiet affair behind the backs of their spouses. They had harbored feelings for each other since school, but their marriage matches had been made before there was time to stop their feelings. So they remained, privately keeping each other company. However, once Lucius had broken out of Azkaban, the two decided it was time to stop hiding and take hold of what they wanted. The Dark Lord took what he wanted, and what was better than to be united in love and suffering for their master? Surely he would look favorably upon them, being such devout followers of his.
On the night of the full moon, Bellatrix and Lucius paid Narcissa a visit. It started cordially with tea and cakes and petty conversation. But with a flick of a wand, that had changed. There was no need for bonds or a restraint, what Bellatrix could do was more than sufficient. Though it took longer than a Killing Curse would, she savored the torture, as she had with every one she had ever performed.
The Cruciatus Curse drove Lucius' wife to the point of insanity, Bellatrix's specialty, and it was all Narcissa could do not to ignore the ornamental dagger that lay before her on the table. She had slashed her wrists while babbling incoherently, the life slowly draining out of her and staining the carpet. As the moonlight streamed through the windows and her body cooled, Lucius took Bellatrix's hand and kissed it.
"My love," Bellatrix smirked triumphantly. "So sorry to hear you're a widower."
"Yes, it's quite tragic." He agreed. "I can't imagine what drove her to kill herself." Lucius added. His lips curled into a dangerous smirk. "Do you think Rodolphus is still awake?"
"There's only one way to find out." Bellatrix laughed.