Clan of the Void

The Dark Shifters are an organization of 51 powerful Shinigami, who, for differing reasons, have banded together to eclipse the Sol Society. Each one of them, at one point, was a Shinigami in the Gotei 13, many of them previously holding the title of Captain, but betrayed the Soul Society, some having more noble or selfish reasons than the others.

History
The Dark Shifters were started by Shikyo Fushiawase, the first Captain of the 2nd Division in the Gotei 13, who went missing many years prior to the start of the series, only for him to later be discovered as the leader of the Dark Shifters. He recruited 31 powerful Shinigami, not the least of which being his own wife, as second in command, Higamippoi Fushiawase. Other than that, much of the history is still unknown, but he claims that he started the Dark Shifters to bring an end to the Spirit King in particular, so that he could take that position, and save the Soul Society from his "unnecesary wrath."

Unique Wardrobe
To stand out, and so peole know who they are, they've acquired a certain wardrobe that is unique to the Dark Shifters. Not the least of which is the cloak they wear, which is pure red, with black lightning bolts on it. In addition, each member of the Dark Shifters have their finger nails painted purple, and if they're shirtless, a tattoo that covers their entire body.

Requirments For Membership
There have been several requirments noted in order to become a member of the Dark Shifters. Shikyo instates these requirments specifically to make sure that each member is worthy of calling themselves a Dark Shifter, either in terms of power level, morals, ect.
 * Bankai Mastery:  This is the most notable trait that they all have--each one of them has to have mastered Bankai.  Shikyo created this requirment so that each one of them will be powerful and a force to be wreckened with, to make their overall defeat much more difficult for the Gotei 13.  It's said that, because of their immense power, if more than five of them attacked at once, they could wipe out the entirety of the Gotei 13.
 * Tragic Pasts:  This one isn't absolutely neccesary, but Shikyo still looks for this.  He looks for people that he can manipulate through their tragic or otherwise unsettling pasts and backstories, so that they'll be deathly loyal to him to their last breath.
 * Loyalty:  Before they're sent out on missions, they're imprisoned until Shikyo can make sure new recruits are deathly loyal to him.  The only ones who escaped this requirment are him, (obviously), Higamippoi Fushiawase, (his wife), as well as Burikka Hakumei Akegata and Zentaitoshitenkushimi Sonodeddo Banbutsunoreichou, because those two were comrades of his back during the days of the original Gotei 13.
 * Morals and Objectives:  This rule can vary the most drastically, but Shikyo states could possibly be the most important one.  Although each one of them could have morals and objectives that differ, (which is something that Shikyo encoruages more than anything), they still all banded together for a common goal, wether they agree with it or not.  Shikyo promises them that their goals will be met if they stick with the Dark Shifters until the joint goal, which is ultimately Shikyo's goal, is achieved.  This is the most key factor to becoming a member of the Dark Shifters, and if they go back on this, then they'll be killed by Shikyo, or someone he appoints to do the ob.

Trivia

 * The Dark Shifters were loosely based on the Akatsuki organization from the anime and manga series Naruto.