Axos

Axos was a scavenger entity which came to Earth, supposedly for fuel. Its real motivations were to drain all energy from Earth to gain the secret of time travel. After its defeat, Axos returned to attack Earth twice.

Biology
Axos was grown from a single cell and had nutrition and energy cycles that needed to be replenished. The eye of Axos was on a stalk that could move, the pupil lighting up when it communicated with its other parts and displaying information. The inside of Axos was covered in claws which could restrain prisoners and could fire long tendrils to drag victims within. It was a biological creature/spaceship; Axonite and the Axons were extensions of it and may have been somewhat independent, though all of Axos was telepathic and each element could feel what the other did.

Axonite
The "chameleon" of the elements, Axonite was a "thinking molecule" in which each molecule was individual, so to speak. It used the energy it absorbed not only to copy but to recreate and restructure any given substance. A lasonic injection of Axonite could enlarge and shrink organisms. It could absorb, convert, transmit, and program all forms of energy (TV: The Claws of Axos) as long as the energy exists. (TV: Doctor Who and the Claws of Axos)

Axons
Axons appeared as golden humanoids, but could transform into large creatures that appeared as a mass of tentacles. They could lash out with their tentacles to either shock or disintegrate enemies. They could also instantly absorb victims. (TV: The Claws of Axos)

Biotechnology
The Axons described their technology as having taken a more organic route, and claimed Axonite to be the source of all their growth technology. Axos also possessed some power of time travel, being able to make time jumps that could only travel moments into the past. (TV: The Claws of Axos)

History
The Axos worlds were on the edge of the Mutter's Spiral. At some point by the 20th century, they were crippled by extreme solar flare activity and entropised, being drained of all life and energy. (TV: Doctor Who and the Claws of Axos) Axos was the last remnant of their culture and was grown for the escape journey. It was a scavenger, sucking energy from planets. Axos encountered and captured his TARDIS, keeping him prisoner as he directed them to Earth, expecting to get back his craft.

In England
On Earth, they tried to start a 'feeding cycle'. They met with UNIT and promised them axonite to enlarge food and end world hunger. UNIT ordered worldwide distribution. The Third Doctor realised the Axons planned to drain the planet of all its energy. They took power from a nuclear power plant and later raided the plant, killing members of UNIT. They cloned Bill Filer for information but the real Bill escaped, preventing this. The Doctor stopped them by placing Axos in a time loop. (TV: The Claws of Axos) Some of the axonite was kept. The Forge had a sample of axonite. (AUDIO: Project: Lazarus)

In Japan
The Axons escaped the time loop and infiltrated 21st century Japan. Disguising themselves as humans, they produced a television programme for children promoting a new soft drink, Goruda. When children drank it, they developed telepathy and were transformed into Axons. Using the children as leverage, Axos convinced the mayor of Tokyo to send it power from a nearby nuclear plant, claiming they only needed enough power to leave. Instead, Axos used the energy to transform into a giant monster, aiming to begin a new feeding cycle.

The Eleventh Doctor defeated it by having the citizens of Tokyo turn on all of their electrical appliances, drawing the plant's energies away from Axos, starving and eventually killing it. (COMIC: The Golden Ones)

The Doctor and Amy later discovered that Chiyoko had freed Axos from the time loop to ensure her own creation. (COMIC: The Child of Time)

Beyond the 21st century
At an unknown time in the future, British explorers tried to break into the time loop Axos was imprisoned in. They wanted to use Axos' power to power the Earth. The mission went horribly wrong and the Sixth Doctor created a nuclear explosion inside the time loop and locked Axos in it. (AUDIO: The Feast of Axos)

Other appearances
At some point, the Eighth Doctor and Mary Shelley fought the Axons. (AUDIO: Mary's Story)

An alien held in Dauntless Prison resembled an Axon. (TV: Liberation)

Behind the scenes

 * Axos have the third longest period before making a return. The Golden Ones was published thirty-nine years after The Claws of Axos. This follows the Macra with a forty year gap, and the Great Intelligence with a forty-four year gap.