(this image appears for illustrative purposes only and no attempt is made to supersede any copyright attributed to it)
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.
In an alternate timeline, while the Doctor still defeated the Empress, he died doing so, leading to a chain of events that drastically altered Earth history. The Doctor's former companion, Rose Tyler, along with the alternate timeline version of Donna, eventually restored the original timeline.