Thirteen

The Thirteen were the first original Cybertronians created by Primus to fight and defeat Unicron.

They were each given the rank of a Prime and were addressed as such.

History
Primus and Unicron were created by a being known as "the One" splitting in two. They began to explore the universe, but unfortunately became divided and began to battle each other. However, as twins they were perfectly matched and neither held an advantage for long, with their battle grinding into a stalemate. Knowing Unicron had to be stopped, Primus created the Thirteen, combining elements of both himself and Unicron to allow for more varied attacks to halt the Chaos-Bringer. The Thirteen defeated him, but at the cost of unknowingly taking a seed of darkness into themselves.

Knowing that one day more Cybertronians would emerge, they established a small city and began working on government. However, as with Primus and Unicron, Prima and Megatronus often clashed. The group began to divide into factions with Liege Maximo creating war beasts in the interim. Eventually all their secrets were discovered and they fought against each other in the conflict that would become known as the War of the Primes. Several perished and/or joined their creator's spark while many departed from the newly-created Cybertron, leaving only Alpha Trion and a few others to help guide the developing Cybertronian society. In time Alpha would be the last true Prime and member of the Thirteen to remain on Cybertron, while his surviving brethren Optimus Prime, Vector Prime, Nexus Prime and Alchemist Prime departed on various self-appointed missions.

Transformers: Robots in Disguise
After the events of Predacons Rising, it was revealed in Decepticon Island, Part 1 that Optimus was saved by the Primes before his own spark could've been extinguished and joined six of his brethren in the Realm of the Primes, which is a mystical plane of existence. Here he trained in preparation to face the threat of Megatronus, who was preparing to destroy both Earth and Cybertron. After undergoing this training for some time, though not enough in the opinion of his brethren, the other six Primes empowered him with portions of their own Sparks to allow him to travel to Earth and challenge Megatronus. Weeks after Megatronus's defeat in Battlegrounds, Part 2 and at the beginning of Overloaded, Part 1, Micronus Prime had to take the powers that were given to Optimus away back to the other Primes, though this left him severely weakened by the resulting power drainage for a while.

Fates
Each member of the Thirteen led a different life following the War of the Primes:
 * Prima led the early Cybertronians and is thought to have died in the Great Cataclysm.
 * Vector Prime left to travel the universe throughout both space and time.
 * Alpha Trion led the early Cybertronians with Prima through their beginning to the Great War and died after the Decepticons' final attack on Iacon.
 * Solus Prime was murdered by Megatronus, an act that made him rename himself "The Fallen", with her body melting away into the surface of Cybertron and opening a channel to the deep core of Primus himself at the center of the planet, now known as the Well of Allsparks.
 * Micronus Prime merged with Primus and would later go on to mentor Optimus Prime.
 * Alchemist Prime left to travel the stars.
 * Nexus Prime split into five lesser bots to protect the pieces of the Blades of Time which could grant access to Vector Prime's personal dimension.
 * Onyx Prime merged with Primus after being grievously wounded.
 * Amalgamous Prime left to travel the stars. He lost his T-Cog and it slowly gave many Cybertronians amazing transformation skills, but at the cost of merging into one being.
 * Quintus Prime created the Quintessons and it is believed he died shortly after teaching them how to make more Quintessons.
 * Liege Maximo was torn apart, decapitated and buried in a poorly built tomb inside the Thirteen's old city. His arm would be later used by Megatron to harness the ancient powers of the Primes.
 * Megatronus Prime murdered Solus Prime, leading to his being renamed "The Fallen". This led to his exile from Cybertron, which he subsequently sought to destroy along with Earth.
 * Optimus Prime chose to be reincarnated as Orion Pax to better understand the lesser Cybertronians and eventually fought in the Great War with them, sacrificing his life to restore the AllSpark for new life on Cybertron.

When creating new life on Cybertron, all humanoid-looking Cybertronians were modeled after certain members of Thirteen, even adopting similar mental and physical characteristics. Alpha Trion believed Megatron, leader of the Decepticons, was modeled after Megatronus, and Nexus Prime noticed the similarity between the two.

Members

 * Prima - Leader, Warrior of Light (Offline)
 * Vector Prime - Warrior, Interdimensional Space-Time Traveler (Fate unknown)
 * Alpha Trion - Strategist, Historian (Offline)
 * Solus Prime - Free Agent, Artificer (Offline)
 * Micronus Prime - Strategist, First Mini-Con (Offline)
 * Alchemist Prime - Free Agent, Elemental (Fate unknown)
 * Nexus Prime - Free Agent, First Combiner, Wizard of Forms (Online)
 * Onyx Prime - Spirit Warrior, First Predacon (Offline)
 * Amalgamous Prime - Warrior, First Shifter (Fate unknown)
 * Quintus Prime - Strategist, Experimental Dreamer (Presumably offline)
 * Liege Maximo - Strategist, Manipulator (Offline)
 * Megatronus Prime/Megatronus/The Fallen - Warrior of Darkness (Presumably offline)
 * Sentinel Zeta Prime - Warrior, Leader, Caste System Creator (Offline)
 * Orion Pax/Optimus/Optimus Prime - Free Agent, Warrior, Visionary (Online)

Trivia

 * On Dec. 10, 2013, Transformers: The Covenant of Primus revealed the origins of the Thirteen.
 * Alpha Trion, Liege Maximo and Prima were the only members of the Thirteen to not have the word "Prime" in their names.
 * It is possible the word and title of Prime was originally derived from the name of Prima, thus Prima can be considered as having an altered version of it as his name. However, it is also likely that the term was also possibly derived from the name of the Thirteen's creator Primus.
 * Alpha Trion is the only member of the Thirteen, except for Optimus to fully appear outside of flashbacks.
 * This is the first appearance of the Thirteen in any fiction. While Vector Prime appeared in Transformers: Cybertron, he was never outright called a member of the Thirteen. The Dynasty of Primes from the live-action film series was a group of Primes as well, only composed of seven members as opposed to thirteen. Another version of the Thirteen would appear a little over a year later in the IDW comics, inspired by their counterparts in the Aligned continuity. So far eight have been confirmed: Prima, Megatronus Prime, Nexus Prime, Solus Prime, Alpha Trion, Liege Maximo, Onyx Prime and Alchemist Prime. The Facebook page Ask Vector Prime and other media indicate that not all the characters presented as members of the Thirteen in the Aligned continuity are members in every continuity.
 * The history of the Thirteen differs on the piece of media presenting it, specifically regarding the Fallen. The novels suggest he and Maximo acted as Megatron and Starscream to the Thirteen and he murdered Solus Prime after a failed attempt to recruit her. Indeed, Alpha Trion often voiced his contempt for Megatronus and Nexus Prime was sent into a blind rage when he mistook Megatron for his ancestor. The Covenant portrays Liege Maximo as the one who truly betrayed the Primes by manipulating the love between Megatronus and Solus. The 2015 Transformers: Robots in Disguise show depicted him as being closer to the novels and second live-action film Transformers: Revenge of the Fallen, but adds that he was banished from the known universe by his fellow Primes and sought revenge on both Cybertron and Earth.