Alrighty, here we go. I can't remember if the back story was made public or not, but I know the higher level missions have NEVER been seen by the general public, not even RoS.
The complete Inferno Mission Series.
In the beginning, Unicron and Primus waged war against each other. Broken and nearly defeated Primus fled to the astral plane, Unicron followed, determined to destroy Primus. Primus however had begun laying a trap, when Unicron followed him to the astral plane, Primus returned to the physical universe where he merged his energy form with a asteroid which he then transformed into a metal world and began to create sentient robotic life upon it. Primus however, was too slow.
Unicron returned from the astral plane sooner than he had anticipated. Seeing what Primus had done, Unicron attempted to merge his own essence with the new planet of Cybertron. It was then that Primus' true plan was revealed, though weakened, Primus used his the remainder of his essence to bind Unicron's essence to Cybertron it's self, though it cost him his own existence, he knew that Unicron was now trapped and would never threaten the universe again.
As millenia passed, the civilisation of Cybertron grew and prospered, everything Primus had dreamed of, his lasting legacy to the universe. What Primus had not foreseen though was the power of Unicron, while he had no body of his own, his essence had spread throughout the entire planet of Cybertron. Seeping into every part of it, while the civilisation grew, so did the dark influence of Unicron. His dark evil essence soon spread to the Cybertronians that Primus himself had created. No one was immune to his dark touch, soon, a planet wide war erupted and the once great civilisation of Cybertron became nothing more than a planet consumed by war fed by the dark influences of Unicron.
Welcome to Hell, prepare for battle.
Limbo
Level 0
5-5
Limbo, the first circle of Hell, the place for those yet to be baptized by the fires of war, those who have only begun to feel the dark influence, it is here where you will begin your journey into the hellish battlefield that is Cybertron, and your descent into hell it's self.
Lust
Level 1-2
5-5
You got a taste of war in Limbo, it left you wanting more, lusting for more. Welcome to Lust, the Second Circle that is the Hell of Cybertron. As the dark influence grows within you, prepare to sate your appetite as for the first time you face those more powerful than you, will it be enough to satisfy you or will you continue to hellish descent into gluttony?
Glutton
Level 2-3
5-5
You have succumbed to the hunger that is war, you've become it's slave. The dark influence drives you, addicted to it you have become a glutton for it. The Third Circle, Gluttony, where you can continue to indulge your hunger for war and the spoils that it brings.
Avarice
Level 3-4
The Fourth Circle, known only as Avarice. The dark influence has begun to consume you, your hunger for war has lead to spoils, energon, weapons, more than you will ever need. You no longer exist to fight, you now crave wealth as well as power. Be wary though, such pleasures will be your downfall and may lead you to unleash your wrath to protect your treasures, or slothful to enjoy them.
Wrath and Sloth
Level 4-6
The dark influence has lead you to the Fifth Circle of hell, it is here you will become wrathful, protecting your spoils of war against others like you, or immersing yourself in them, foregoing your war like nature to become lazy, indulgent, sloth like. But how long can you ignore the feel of the dark influence as it drags you deeper into the depths of hell?
Heresy
Level 5-7
Your greed for the spoils of war and your indulgence of them have lead you away from the path of the dark influence, such heresy will not be tolerated! Feel the dark influence seep deeper into your spark, feel your spark begin to burn, trapped in the tomb that is your heretic body, give in and let the dark influence consume you completely so that you may descend into the seventh level of hell.
Violence
Level 6-9
Every fibre of your being is now tainted with the dark influence, driven by it's desires, unleash your true nature in the seventh level of hell, Violence! Let none stand in your way as you cut a swathe of destruction through those that stand between you and ultimate power.
Fraud
Level 7-9
Consumed by the dark influence, you now see the truth, or the truth as you like to believe. There was no dark influence, the urge for war and violence was always within you and all other Cybertronians. This is the eighth level of hell, that of Fraud, where everything you believed, everything that drove you, is nothing more than a lie. There is only one thing that can appease you with this revelation, lash out, lash out and destroy everything!
Treason
Level 9-11
Heathen! You dared to not believe the dark influence? You dared to believe only in yourself? Only someone with such treasonous, such blasphemous thoughts could descend here to the night level of hell. Let the truth be revealed to you, cast your eyes on the dark influence himself, behold Unicron! Succumb to him, do his bidding, destroy those who oppose you so that you may become his vessel so that he may once again be free, unleashed to destroy the universe!
Hell Unleashed
1-10
Primus failed. Unicron has been reborn! With his dark influence still touching your spark, you have no choice but to give in to the desires, whether you just begun your baptism of war, or if you've been to the ninth level of hell it's self, Unicron only cares for one thing, destruction of all! Fight or perish, only the strongest will be allowed to survive, and only the strongest will be able to resist the dark influence and rise up to do that which Primus failed to do.
I copied a Decepticon mission titled Inferno: Limbo and made a Transformers Mosaic like comic strip out of it. I'd love to hear anyones thoughts - and I know it should be in the creative section but I wanted to post it here first as it deals with HMW. 
