The Dark Ones are a clan of Shinigami that were formerly known as the Fallen House of Ryodoji, who were exiled over a millennia in the past and the villains from Bleach: Memories of Nobody.
More than 1000 years ago, the House of Ryodoji were banished to the Dangai Precipice World for reasons unknown. However they survived for a long time, avoiding destruction at the hands of the deadly current present in the Dangai. The Dark Ones eventually discovered the Blanks, lost souls that had lost their memories and been unable to transmigrate to the Soul Society. They trained themselves to harness the Blanks as a source of power and learned everything they could about the wandering souls, as well as the Valley of Screams where they took up residence. After finding out about the Shinenju, a powerful object formed from the lost memories of the Blanks, the Dark Ones planned to use it to take their revenge on the Soul Society by crashing it and the Human World together, destroying both. They went to the Human World to find the Shinenju, eventually finding it had taken the form of a Shinigami named Senna. The Dark Ones managed to capture her despite interference from the other Shinigami and brought her to the Valley of Screams to enact their plan. But the Shinigami arrived and defeated the Dark Ones, stopping their plans.
The Dark Ones are a skilled group of warriors with each one able to use a different weapon and able to work well as a team. However their greatest ability is their manipulation of the Blanks, which they can form into weapons or even to regenerate themselves.