The Eight Legs were a race of giant spiders with psychic powers.
Characteristics
Physical characteristics
The Eight Legs were black spiders, generally less then a metre in length, though the Great One was much larger. They relied on human mounts for transport, though they could scuttle along at walking pace. The Eight Legs could create webs like the spiders of Earth, though they were not sticky, merely tough. They fed on the flesh of living creatures, generally the sheep of Metebelis III, though they preferred human meat. (TV: Planet of the Spiders)
Powers and abilities
The Eight Legs could communicate through telepathy, discharge psychic energy, use mind control on humans, and teleport through both time and space. To teleport greater distances required more spiders to focus their wills.
They could ride on the backs of humans, turn invisible and possess the human who could act as a vessel for the Eight Leg's power. They could project psychic energy blasts, the strength of which varied according to the strength of the victim. (TV: Planet of the Spiders)
History
A starship of human colonists crash landed on Metebelis III with Earth spiders on board. In the mountains, the native blue crystals caused the spiders to grow in size and mental power. Over the course of millennia, the spiders enslaved the colonists, forced them to harvest food and ate the humans themselves. They forbade the humans to use the word spider, referred to themselves as Eight Legs, and called the humans Two Legs. The Eight Legs developed a matriarchal society centred around the physically enormous Great One.
The Eight Legs planned to conquer the rest of the universe with the Great One planning to collect all of the blue crystals, believing it would give her immense powers. Though they were nearly successful, the final crystal was held by the Third Doctor on Earth. The Eight Legs sent an expedition back in time to obtain the final crystal. Though the crystal was returned to them, the power of the crystal web was too much to contain. The energy feedback destroyed the Great One. (TV: Planet of the Spiders)
Despite this, some of the Eight Legs escaped to Earth, by travelling down the ley lines. The Fourth Doctor and Romana II encountered the Eight Legs living in an isolated house, hiding away and supplying food for their queen, while she gave birth to dozens of unhatched Eight Legs to take over the world.
The Doctor stopped the Eight Legs' plans by setting fire to the house, alerting the fire brigade. As the eggs began to hatch, the Doctor used the high powered jet of water from the houses to kill all the Eight Legs in the house. (PROSE: Return of the Spiders)
Iris Wildthyme encountered Giant Spiders on Metebelis III with her companion Timmy. (PROSE: The Scarlet Empress)
Some of the Eight Legs traveled back in time and tried to use a Time Lord stellar manipulator to invade Earth in 2015. They were stopped by the Eighth Doctor. (AUDIO: The Eight Truths)
When Es'Cartrss tried to take over the Tenth Doctor's mind via the TARDIS' Matrix, he used his memories of the Eight Legs to attack him. (COMIC: The Forgotten)