Terileptils were a space-faring reptilian species, known for a love of art and beauty, matched only by their love of war.
Biology
Terileptils were reptilian humanoids, but also had fish-like qualities. Each Terileptil seemed to be of primarily one colour — for example, green, blue or red — but some of their scales and ruffles had variations in hue.
Terileptils breathed through gills on the side of the head, their hands and feet were like flippers, and their wide mouths were full of pointy teeth. They breathed soliton gas. (TV: The Visitation)
Technology
Terileptils had a highly sophisticated technology. Dependent on soliton gas, they perfected atmospheric processors which could synthesise and release the gas into the immediate area, allowing them to function in alien atmospheres lacking this vital component.
They were familiar with the use of Vintaric crystals as a means of artificial lighting, and holographic energy barriers. Terileptilan technology made heavy use of polygrite and highly charged power packs. They made frequent use of androids as guards, soldiers, and servitors. (TV: The Visitation)
Culture
The Terileptils placed great value on beauty, using aesthetically pleasing designs even in weaponry. Even the Terileptilan language reflected the artistic and scientific sophistication of this species; to the untrained eye Terileptilan script looked very much like mathematical formulae. As a part of this highly refined culture, the Terileptils esteemed certain vocations, such as mime and circus clown. They considered war an honourable activity and that it stood side by side with art and beauty.
Terileptils were allied with the inhabitants of the planet Hakol in the Rifta system, for whom they mined tinclavic, which was found on the planet Raaga. Condemned criminals were conscripted to mine this mineral, overseen by android guards and wearing control bracelets. Severe facial scarring was a common feature of these conscripts, possibly due to the toxic nature of raw tinclavic ore. (TV: The Visitation)
History
In 102, the Terileptils were among the races who joined the Alliance. They came to Stonehenge and helped imprison the Eleventh Doctor in the Pandorica in order to save the universe. (TV: The Pandorica Opens)
As early as Earth's late Middle Ages, Terileptils mined tinclavic for the people of Hakol. A psychic probe from Hakol landed on Earth at this time, becoming briefly active in the year 1643 and again in 1984. (TV: The Awakening)
In 1666, the Fifth Doctor found a small group of Terileptil criminals who had escaped from the penal colony on Raaga. They had somehow managed to override their control bracelets, reprogram an android guard to help them, and steal a ship. They were caught in an asteroid storm crossing through Earth's planetary system. The ship's escape pod crashed outside a small village near the future location of Heathrow Airport, with only four Terileptils surviving the crash. The Doctor could not return the creatures to their home planet, because Terileptil law stated that imprisonment on Raaga was for life. Plague was already raging in London, and the three remaining Terileptils began to genetically augment the plague bacteria, plotting to eradicate humanity and seize control of the planet.
Their plot failed when the Doctor and his companions, along with Richard Mace, fought them in a bakery on London's Pudding Lane, and a burning torch dropped during the scuffle, together with an overloaded Terileptil hand weapon, resulted in the Great Fire of London, which killed the Terileptils and destroyed their augmented plague bacteria. (TV: The Visitation)
In 3375, the Adjudicator Terrell and the Master destroyed the Terileptil homeworld using the Darkheart device. Aside from individual Terileptils who may have been elsewhere in the galaxy when this occurred, the species was wiped out. This act ended an interplanetary civilisation more than two thousand years old and was also one of the final steps in the Master's descent into darkness. (PROSE: The Dark Path)
Undated events
At some point in their history, the Terileptils went to war with the Torodon. (PROSE: Hunter's Moon)
Terileptils were present during the Siege of Trenzalore. They either retreated or died in the fighting which took place after the Daleks broke through the Papal Mainframe's force field and plunged Trenzalore into war. Their forces either retreated or were destroyed before the final battle between the Eleventh Doctor and the Daleks. (TV: The Time of the Doctor)
A Terileptil was a customer at the Intergalactic Coffee Roasting Station. (PROSE: Lights Out)
Alternate timeline
In an alternate timeline created by the Black Guardian where the First Doctor never left Gallifrey, and became Lord President, the Terileptils were one of many aliens that invaded Earth, and fought over the planet with other races against the Silurians and the Sea Devils. This timeline was destroyed when the Seventh Doctor retrieved the Key to Time. (COMIC: Time & Time Again)
References
Tegan and Nyssa saw a hallucination of a Terileptil projected in an attempt to prevent them from entering the Inner Sanctum presided over by Kalid. (TV: Time-Flight)
The Sixth Doctor had a nightmare in which he and Tegan visited Raaga and helped Terileptil slaves construct a time corridor and escape. (PROSE: CHAOS)
The Yssgaroth's universe contained the remains of Terileptils who had been brought there. (PROSE: The Pit)
The Seventh Doctor had a Terileptil sculpture in his TARDIS art gallery. Ace thought it looked like a glass factory exploded. (AUDIO: Dust Breeding)
The Twelfth Doctor blamed the Terileptils for the Great Fire of London. (TV: The Woman Who Lived)
River Song mentioned that Terileptils were part of the invading forces in the events of the Pandorica. (TV: The Pandorica Opens)
Other information
When cyborg hacker Psi was attempting to protect Clara Oswald from the Teller, he ensured that it would focus on his guilt by uploading the memories of some of the worst criminals in history, one of them being a Terileptil. (TV: Time Heist)
Behind the scenes
The Terileptil illusion seen in Time-Flight part two was played by Chris Bradshaw, who remained uncredited both on-screen and in Radio Times. (DWM 294)