Terileptil

Terileptils are a space faring reptilian species. They are well-known for possessing a great love for art and beauty which is matched only by their great love of war.

Technology and culture
Terileptils possess a highly sophisticated technology. Dependent on Soliton gas, they perfected atmospheric processors which can synthesize and release needed amounts of the gas into the immediate area, allowing them to function in alien atmospheres lacking this vital component. They are familiar with the fairly common use of vintaric crystals as a means of artificial lighting, and the use of holographic energy barriers. Terileptilan technology makes heavy use of polygrite and highly charged power packs. They do not appear to have knowledge of time travel, but are certainly familiar with the concept. Terileptils are especially noted for their use of androids, as guards, soldiers, and servitors. Like their weapons and indeed all their technology, these androids are impressive in their aesthetically pleasing design. Even the Terileptilan language reflects the artistic and scientific sophistication of this species; to the untrained eye Terileptilan script looks very much like mathematical formulae.

Terileptils are allied with the inhabitants of the planet Harkol in the Rifter system, for whom they mine tinclavic, a key source for which is found on the planet Raaga. Condemned criminals are conscripted to mine this mineral, overseen by android guards and wearing control bracelets. Severe facial scarring is a common feature of these conscripts, possibly due to the toxic nature of raw tinclavic ore.

History
Terileptils became a space faring race centuries ago. At least as early as Earth's late Middle Ages, they were mining tinclavic for the people of Harkol. A psychic probe from Harkol is known to have landed on Earth at this time, becoming briefly active centuries later in the year 1643 and again in 1984 ("The Awakening").

The Doctor was certainly familiar with Terileptils and their technology prior to his fifth incarnation. In 1666 he found a small group of escaped Terileptil criminals from Raaga, who had somehow managed to override their control bracelets, reprogram an android guard to help them, and steal a ship. The ship was caught in an asteroid storm crossing through Earth's planetary system and the Terileptil's escape pod crashed outside a small village near Heathrow. An outbreak of plague was already raging in London, and the 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 fought them in a bakery on London's Pudding Lane and a torch dropped during the scuffle, together with an overloaded Terileptil hand weapon, resulted in the Great Fire of London, which destroyed the Terileptils and their augmented plague bacteria ("The Visitation").

The Expanded Universe
The Missing Adventures novel, "The Dark Path" names the Terileptils' home planet as Terileptus, and places it near the Darkheart colony, between two of the spiral arms of Earth's galaxy, beyond Lasky's Nebula. Adjudicator Terrell and Koschei, the Time Lord later known as the Master, destroyed their homeworld using the Darkheart device. This occurs some 350 years after the fall of the Earth Empire, during the early years of the Federation. Aside from individual Terileptils who may have been elsewhere in the galaxy when this occurred, this act marked the end of an interplanetary civilization which was over two thousand years old, and was also one of the final steps in the Master's descent into darkness.