The Empress of the Racnoss was the leader of the Racnoss.
When the Racnoss Empire was destroyed by the Fledgling Empires, the last remaining Racnoss hid themselves by forming the core of what would become the planet Earth. The Empress was the only Racnoss to survive and not hibernate in the Earth's centre. Instead, she hid at the edge of the universe for billions of years until Torchwood uncovered the Secret Heart, the ship her children were in and developed Huon particles which she could use to reawaken them. The ship then sent a signal to her Webstar which drew her to Earth.
The Empress met the Tenth Doctor on Christmas Day 2007, in a secret location under the Thames Flood Barrier. She was trying to awaken her long-dead race, whom she called her children, with the Huon energy implanted into the body of Donna Noble by her fiancé, Lance Bennett. Lance was originally affiliated with the Empress; however, when Donna escaped with the Doctor in the TARDIS, the Empress decided to use him instead. She force-fed him Huon particles and dropped him into the centre of the Earth, where the Racnoss spaceship was. There, the Huon particles awoke the Racnoss, who feasted on Lance.
Her plans to revive the Racnoss were foiled by the Doctor. When she ordered the Robot Santas to kill him, he used stolen controls to order them to "relax". He set bombs, disguised as baubles from a robotic Christmas tree, flying around the secret room. The bombs detonated, crashing into pipes, letting the Thames flood into the room. The Empress was surrounded with fire, and the awakening Racnoss were drowned with the Thames water.
The Empress transported herself to her Webstar and vowed revenge on the Doctor and all of planet Earth by scorching it. However, the lack of Huon energy left the Webstar defenceless, and the British Army destroyed it along with the Empress. The order to destroy the Webstar came from Harold Saxon himself. (TV: The Runaway Bride)
The Empress was a very ferocious and feral Racnoss and was prone to roaring, growling and hissing loudly at her enemies, possibly as a natural habit or simply to unnerve them. Being the mother and leader of an omnivorous race, she was very arrogant and immediately made it clear that she viewed all other races as prey and food for her species.
She held great affection for her children and set in motion an elaborate plan to revive them, showcasing her intelligence. She was distraught when they all died at the Doctor’s hands and swore to burn the Earth in revenge.
One of her most prominent traits was her predatory sadism. Plagued by a ravenous hunger common to all Racnoss, the Racnoss frequently taunted and mocked her foes the way an animal plays with its food. This sadistic streak seemed particularly directed towards males of any species, whom she considered “funny”.