Sentient oil

Sentient oil was a component in some space ships. As a ship's systems, it served as engine, fuel, and oil all at once, and was possessed of a simple artificial intelligence. (PROSE: Philology: The Real Professional Bag of Tricks) The oil was able to mimic other beings, survive a blast from a Dalek gun, target passengers and follow them across time and space. (The Doctor Falls)

History
A puddle of sentient oil was left behind by a spaceship that had previously landed in Bristol.

Heather had previously come across the puddle around St Luke's University, noticing something strange about her reflection. She took Bill with her to show her the puddle, and Bill knew something was different but couldn't tell what.

Bill told the Twelfth Doctor about the puddle, and before she could finish explaining, the Doctor had already begun to run to the site. The Doctor quickly discovered what was off about the reflection, which was that it wasn't reversed horizontally like a mirror should be. The Doctor also noticed scorch marks in a circular pattern surrounding the puddle, but claimed he was bored to lead Bill away. As they left, the oil started creeping away from its spot. Bill later saw the oil in her bathroom, now taking the form of Heather.

Bill ran to St Luke's only to find the oil there again, repeating everything Bill was saying to her. Bill ran into the Doctor's office and shut the door behind her, locking it in place with a chair. The puddle managed to flow underneath the door and reappear as Heather again. The Doctor led Bill into the TARDIS and travelled to the vault. After realising that the oil was after them and not the vault, they ran back to the TARDIS and landed in Sydney, where the oil appeared as well.

Determined to find the limitations of the oil, the Doctor travelled 23 million years into the future onto another planet. The oil followed them through time and space to this planet as well. The Doctor and Bill went back into the TARDIS and landed in the middle of a Dalek-Movellan War to get a Dalek to exterminate the oil.

A Dalek shot the oil but it easily survived the attack, repeating the Dalek's "exterminate" phrase whilst doing so. The Doctor and Bill were trapped by a Dalek but it was revealed to be the oil having taken the form of a Dalek, although with Heather's star eye. It transformed again into Heather. The Doctor realised that the oil didn't want to kill, but wanted Bill to travel with it. Bill remembered that she asked Heather to promise that she wouldn't leave her, so this was the last thought Heather had before being eaten by the oil. Bill told the oil to let her go, the oil resumed its puddle form and remained at the war zone. (TV: The Pilot)

Bill later recalled this vessel as "the Heather Creature", after the shape it took. (TV: The Lie of the Land)

After the Doctor blew up Floor 0507 of the Mondasian colony ship, Heather returned and turned the cyber-converted Bill into a being with the same properties as her. In this state, as "the Pilot", Heather possessed the ability to fly the TARDIS and even return Bill to being human if she so chose. After saying a tearful farewell to the Doctor, Bill left with Heather to explore the universe. (TV: The Doctor Falls)

Sentient oil was also a component of the Jama'a flagship. When the ship was floundered, the oil offered the Jama'a a deal for their "survival." It was a ruse to take their civilization for the oil's own end. The oil imitating the Jama'a flourished across a galaxy for centuries, before finally dying in war. PROSE: Philology: The Real Professional Bag of Tricks

Characteristics
Sentient oil changed its molecular structure and shape depending on what was required of it, and could serve as anything from ship's systems to fuel or lubrication. Its artificial intelligence was diffused throughout the whole of the oil in a ship. Despite being a simple AI, it could communicate with its pilot on at least a basic level. (PROSE: Philology: The Real Professional Bag of Tricks) It could easily traverse space and time. It could survive immense damage (taking Dalek fire in stride), and follow a target across space and time. When taking the form of a human, the being appeared wet. (TV: The Pilot) Sentient oil taking human form was not human enough to be resurrected in the City of the Saved. (PROSE: Philology: The Real Professional Bag of Tricks) Olew ymdeimladol