Eight Legs

The Eight Legs were giant spiders with psychic powers.

Physical characteristics
The Eight Legs were black spiders, generally less then a metre in length, though the Great One achieved a much larger size. The Eight Legs 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 and are merely tough. They also fed on the flesh of living creatures, generally the sheep of Metebelis III, though they preferred Human meat. (DW: Planet of the Spiders)

Powers and abilities
The Eight Legs could speak through telepathy, discharge psychic energy and teleport through both time and space. To teleport further 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, for example, project psychic energy blasts, the strength of which varied according to the strength of the victim. (DW: 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 both in size and mental power. Over the course of millenia, the spiders enslaved the colonists, forced them to harvest food and ate the Humans themselves. They forbid the Humans to use the word spider, referred to themselves as Eight Legs and calling 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 Doctor on Earth. The Eight Legs sent an expedition back in time to obtain the final crystal. (DW: Planet of the Spiders)