The Quintessons were introduced to the Generation One cartoon continuity in Transformers: The Movie. Living on the twisted mechanical world of Quintessa, they sought to conquer Cybertron and take revenge upon all Transformers. As fate would have it, it turned out that the Quintessons were responsible for creating the Transformers and building the planet Cybertron, which was originally nothing more than a colossal factory. They created the Autobots as labourers and servants while the Decepticons provided amusement to their masters as gladiators who would fight to the death. The Quintessons never intended for their creations to develop emotions, but the time eventually came when Autobots and Decepticons refused to suffer for their makers' enjoyment and rose up against them. They learned the art of transformation as a means of survival and used it to their advantage in driving the Quintessons off Cybertron. However, the Quintessons would spend the next several million years nursing a bitter grudge against their rebellious creations and would finally take action against them following the destruction of Unicron. They concocted various schemes with which to re-conquer Cybertron and annihilate all Transformers, often using deceit and trickery in favour of straight-forward aggression. When they first took action against the Autobots, they had captured Rodimus Prime and his closest companions and brought them to Quintessa in order to destroy the Matrix of Leadership. The Autobots attempted to escape and the Quintessons even went so far as to blow up their own planet to ensure the Matrix's destruction. This act of desperation was for naught, however, as the Autobots escaped just in time.
Marvel comics continuity
In 2008, Quintesson scientists realized that their planet, Quintesson (not Quintessa), was threatened with destruction by a time rift that was caused by Galvatron, Cyclonus and Scourge's time-traveling.
Desperate for their race to survive, they launched a surprise attack on Autobot City, capturing it for themselves. Most of the Autobots were deactivated, except for Rodimus Prime, who was outside the city at the time of the attack, and Rewind, Ramhorn and Steeljaw, who were hidden inside Blaster's cassette compartment.
Using Arcee as bait, the Quintessons captured the Creation Matrix from Rodimus, and he reverted to Hot Rod. Out of options, Hot Rod activated the core of Autobot City, a giant Autobot called Metroplex, who transformed and drove the Quintessons off but caused massive damage to the city in the process.
The Quintessons fled, but were ordered by Lord Kledji not to return to Quintesson, but to roam the stars instead to rebuild their forces and seek revenge on the Transformers, since their home planet was doomed beyond hope.
JudgesThe most prominent and famous type of Quintesson, as well as the apparent "leaders" of the race, are the Judges, also referred to as the Imperial Magistrates. Judges have inverted, egg-shaped bodies which float on a beam of energy and possess wiry tentacles for physical manipulation, but their most distinctive physical characteristics are their five rotating faces. Each face represents a different aspect of the Quintesson, though for four of the five faces, there are conflicting accounts as to what each represents.
BailiffsBailiffs are roughly humanoid with plated armour skin and snouted faces. They serve as guards in the Quintesson Courts, protecting the Judges and dragging prisoners from their cells to receive sentences.
GatekeepersGatekeepers - also known as "Executioners" - are hunchbacked, humanoid Quintessons with spikes adorning much of their upper bodies. They are the slave drivers in Quintesson society, torturing victims with electrified pikes and are responsible for feeding criminals (whether guilty or innocent) to the Sharkticons.
ProsecutorsProsecutors, like Judges, float on beams of energy as their primary locomotion method. They have large, wedge-shaped heads and three tentacles protruding from each "arm" socket. They act as sub-commanders and carry out the will of the Judges, asking them for their verdicts in the Quintesson Courts.
Scientists are somewhat similar to the Prosecutors in construction, though they feature more bulbous heads and bearded, somewhat-more-human faces.