That which does not become a part of the one, shall become one with the void.
Unicron is the ultimate antagonist in the Transformers franchise. He is a cosmic deity, and a giant Cybertronian god who serves as a multiversal singularity desiring to find peace by consuming and destroying all reality around him. He is also the twin brother and arch-nemesis of Primus, the creator of the Transformers.
In his original, divine form, Unicron is depicted as an armored warrior, including a horned helmet, and wielding a spiked mace. This is exclusive to the G1 Marvel comics.
Unicron has a robotic version of pretty much the same shape - humanoid, but made of metal, with horns and spikes and massive skeletal wings. In robot mode, Unicron is more than half as tall as Cybertron's diameter; even in the smallest version, therefore, he is about a thousand miles in height, while taken as the largest version he would stand over forty thousand miles tall.
His planet mode looks like a giant, mechanical world with a metal ring and a circular mouth with immense, horn-like crushing mandibles, capable of tearing a planet's crust open. His size as a living planet is a matter of contention, but he is typically about a quarter of the diameter of Cybertron which, depending on the version, is the size of Saturn or smaller than Earth's moon.
Unicron is the eternal archenemy of his twin brother Primus. He is known as the Dark God, the Chaos Bringer, and the Planet Eater, and he is dedicated to consuming the multiverse. His goal is to bring an end to the annoying creation boasting independence around him, and find peace by becoming the living center of a swirling, infinite torrent of nothingness at the end of all things. To undertake this seemingly overwhelming task, Unicron is able to travel across realities at will, a meandering plague upon existence itself. Integrated into his systems are incomprehensible quantum computers which calculate probabilities forward and backwards in time, in perpetuity, giving his processors an ever changing, evolving map of the multiverse.
The only thing that he fears is Primus' essence, contained inside the Matrix.
Frequently, Unicron will make deals with lesser beings, giving them vast new powers in exchange for their servitude. These minions are sometimes stripped entirely of their free will, but others follow him willingly. However, Service to Unicron is a double-edged sword, for it causes insanity and loss of self. And in the end, no matter their intent, Unicron's plans ultimately call for their consumption as well.
Unicron fulfills his mission one world at a time, by physically travelling though space - at speeds greater than light speed - and consuming planets, preferably life-bearing ones, and breaking them down to energy, so that there is not even debris left. Sometimes, he transforms former denizens of planets and gives them immense additional powers; but they are Unicron's slaves, constantly under his control. Once Unicron has emptied a universe of life and matter - it is intimated he can even feed on stars - he moves on to the next, and he will continue to do this until all that is not part of him has become a part of the void - has become nothing.
Powers and Abilities
Master Manipulation: Unicron is able to manipulate other beings by making unfair deals with them, and giving them powers. However, he also desires to consume them.
Transforming: Unicron is able to transform himself. He is a deity, but can turn into a giant mechanical robot which can transform into a world which consumes other worlds.
Consumption: Unicron, in his planetary form, can consume entire worlds and all the life living on them. In The Transformers: The Movie, he was also able to consume Galvatron in his robotic form, simply by picking him up and dropping him into his mouth.
Immortality: Due to him being a deity like Primus, Unicron is immortal, and cannot die of old age.
Megatron… Megatron… welcome, Megatron! (Megatron: Who… who said that?) I… am Unicron. (Megatron: Show yourself!) I have summoned you here for a purpose. (Megatron: No one summons Megatron!) Then it pleases me to be the first.
~ Unicron meeting a badly wounded Megatron.
This is my command: you are to destroy the Autobot Matrix of Leadership. It is the one thing, the only thing that can stand in my way. (Megatron: You have nothing to fear. I've already crushed Optimus Prime with my bare hands!) You exaggerate. (Megatron: The point is he's dead, and the Matrix died with him!) No, the point is you're a fool! The Matrix has been passed on to their leader… Ultra Magnus. Destroy it for me.
~ Unicron giving Megatron a new assignment.
Your bargaining posture is highly dubious… but very well, I will provide you with a new body and new troops to command. (Megatron: And?) And NOTHING! You belong to me now. (Megatron: I belong to NOBODY!!) Perhaps I have misjudged you. Proceed… on your way to oblivion. (Megatron: AAAAAH!!! NO, NO!! I ACCEPT YOUR TERMS!!! I ACCEPT!!!) Excellent!
~ Unicron forcing Megatron to swore an allegiance to him.
Behold… Galvatron! And these shall be your minions… Scourge the tracker, and his henchmen, the Sweeps. Cyclonus the warrior, and his armada. And this… shall be your ship. Now go… destroy the Autobot Matrix. (Galvatron: I will rip up Ultra Magnus and every other Autobot until the Matrix has been destroyed! To Cybertron!) Destroy the Matrix!
~ Unicron transforming Megatron into Galvatron before sending him and his forces off.
(Galvatron: Unicron! UNICRON! See this? The Matrix! I now have that which you most fear! You will do my bidding, or TASTE MY WRATH!) You underestimate me, Galvatron.
~ Unicron berating Galvatron for his attempted betrayal.
For a time… I considered sparing your wretched little planet Cybertron… but now, you shall witness… ITS DISMEMBERMENT!!
~ Unicron to Galvatron before attempting to destroy Cybertron.
Destroy him, Galvatron. NOW!! OR YOU YOURSELF SHALL BE OBLITERATED!!
~ Unicron mentally torturing Galvatron to kill Hot Rod.
Destiny… you cannot destroy… MY DESTINY!!!
~ Unicron's last words.
Although Megatron/Galvatron is responsible for driving the entire plot of the Transformers series, Unicron is shown to be the most dangerous villain, because he would choose to destroy Cybertron (and all reality) rather than taking it over.
Although Unicron was originally going to appear in Transformers: Age of Extinction as the main antagonist, he was replaced by Harold Attinger. However, Unicron definitely exists in the Transformers film series, and could still very well appear in the next movie.
It is unknown what Unicron's origins are in the Transformers Cinematic Universe, either being a giant robot, or an evil deity. However, due to Unicron being the core of Earth (and Primus being the core of Cybertron) in Transformers: Prime, the latter seems more likely to be the case.