Two species, one flesh, one metal. From two different worlds, yet they only come to one, out of the one hundred billion trillion planets in the cosmos. You want to know, don't you, why they keep coming here? 

In order for my world to live, yours must die. The planet you call earth is corrupted, by the evil that plagued my world before our creation, he had a name before this one that you gave him, his name is Unicron. And I shall be the one to destroy him, and no one shall stand in my way of my mission