Axos was a composite creature, a scavenger entity which came to Earth, supposedly for fuel. Its real motivations were to drain all energy from Earth, with an ultimate plan to gain the secret of time travel, allowing it to feast anywhere in space and time. After its defeat, Axos returned to attack Earth twice.
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 were independent, though all of Axos was telepathic and each element could feel what the other did. (TV: The Claws of Axos)
It used thought windows to see the different sections of itself and would bleed if cut into. As Axos absorbed energy directly, individuals walking within it would give it more power the faster they moved. (AUDIO: The Feast of Axos)
By its nature, Axos was morphologically unstable living organic matter, which meant it was possibly capable of performing Block Transfer Computations and surviving the stress. (PROSE: Cat's Cradle: Witch Mark)
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. (PROSE: Doctor Who and the Claws of Axos) Additionally, individuals could be copied to obtain their knowledge, but some objects were too complex to be copied. (AUDIO: The Feast of Axos)
Axons appeared as golden humanoids with white bodies covered in gold blotches. Their skin was metallic gold, with curly hair and "covered" eyes. They could transform into monsters. They could lash out with their tentacles to either shock or disintegrate enemies. They could also instantly absorb victims. (TV: The Claws of Axos) The Sixth Doctor called these transformed creatures "Axonoids" while Axos referred to them as "Axos units." (AUDIO: The Feast of Axos)
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) It could incorporate alien technology into itself (though some was not compatible) and improved the technology with Axonite. (AUDIO: The Feast of Axos)
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. (PROSE: 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 the Master and captured his TARDIS, keeping him prisoner as he directed them to Earth, expecting to get back his craft.
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. The Doctor stopped their plan by placing Axos in a time loop. (TV: The Claws of Axos)
A fragment of Axos was freed from the time loop by Chiyoko to ensure her creation, (COMIC: The Child of Time) and infiltrated 21st century Japan. Disguising themselves as humans, the Axons 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 Axos 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)
Beyond the 21st century
In the mid-21st century, British explorers tried to break into the time loop Axos was imprisoned in, hoping to use Axos' power for a new age of energy for the Earth. Axos turned their transmitter into a receiver and began feeding on Devesham. It copied the Sixth Doctor and made him dematerialise the TARDIS out of the time loop as Axos would follow. Two of the explorers set off a nuclear explosion with their ship, dying for nothing, before the Doctor used the fast return switch to send Axos back. Separated from its main body, the Axon copy of the Doctor melted; Axos would remain trapped in and feeding on the explosion until the time loop decayed naturally, sometime six billion years in the future. (AUDIO: The Feast of Axos)
An episode of Doctor Who entitled The Claws of Axos was broadcast in 1971. In 2013, Claudia Winkleman mentioned the serial and guessed (incorrectly) that the suprise that her co-star Dermot O'Leary got her was an alien duplication unit from the story. (TV: The Doctor Appears)