The Xhinn were a race of conquerers who travelled from planet to planet, stripping each of resources to conquer more worlds. They were aware of the Time Lords but considered them an impotent and weak race though technologically advanced.
Xhinn were roughly humanoid, though they were described as being made of darkness and light. They had tentacles instead of hair and hundreds of eyes on their faces. They didn't have any mouths. They communicated telepathically. Though Xhinn had extremely long lives, they could increase them by absorbing energy from other Xhinn. They were also highly flammable.
Xhinn had a variety of advanced powers. They could shoot blue beams of energy from their hands and teleport anyone hit by this energy. They could regrow limbs and control lost limbs from a distance. They could levitate, and preferred it to walking. They could also become exact copies of humans in appearance and memories.
The Xhinn travelled in shadowy spacecraft which were nearly impossible to track.
During their invasion of London, they added a special chemical to the fog to poison and kill the inhabitants. They also used a special drug to control the minds of humans. This drug was so effective that if a human were fatally wounded, the drug would keep them moving.
Legends state that five million years ago, the Xhinn were created out of fire in an alternate universe. They rebelled against their creator and were banished into the universe. They started on a divine mission, led by the Triumvirate, to have the entire universe worship them as gods. They were a highly methodical species, spending years studying species in order to find their weaknesses.
In 1900s, the Xhinn sent a scout to Earth to learn about the planet. They sent two Xhinn to join the humans. One of these scouts ignored the plans in 1952 and forced them to poison the fog of London to cover up their tracks. These attempts were stopped by the Third Doctor and Sarah Jane Smith with the help of the local gangsters. The Third Doctor created a Time Bomb. It aged the Triumvirate past their natural life-span and destroyed their spaceship.