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The Darkest Hour
When IDW's Transformers line of books began, main architect Simon Furman consiously chose to ignore the traditional Primus and Unicron mythology that he himself had created while penning the original Transformers Marvel comic, mainly due to the Unicron Trilogy of cartoons featuring the two deities predominantly. Following Furman's removal however, various authors would occasionally reference Primus but merely as a religious belief. Unicron was never even referred to by name, only being known as the "opposite" of Primus.
The proper build-up began in issue #34 of Robots in Disguise which saw the introduction of IDW's Thirteen and served as a stepping stone for more traditional Cybertronian mythology into IDW's universe. Issue #10 of Optimus Prime would later speak of the ancient Titans doing battle with "Primus' opposite", with Unicron making his first physical appearance on the final page of First Strike two months later.
Awakened by the invasion of Cybertron, Unicron has begun to devour the galaxy, starting with...Elonia? The Transformers race to defend the planet and find out why the Chaos Bringer has chosen it as his first target on the road to their homeworld.