Unicron, I mean. I haven't made many Transformers: Prime posts, and I've certainly not made one recently. The show, which started almost a year ago, is now in the final 3-parter of its first season (and still not a single toy on shelves until maybe next month...).
Anyhow, the 3-parter (One Shall Rise or whatever) is all about the return of Unicron to the small screen. In this universe, dark energon is the blood of Unicron, and Megatron ended up finding a literal mother load of the stuff on Earth (he thought he'd have to go out into space to get more). This is because, at Earth's core, there's Unicron. He's been taking a dirt nap (ok, he wasn't really dead) for billions of years. After his defeat by the 13 Original Primes/Transformers, he apparently ended up floating off near the Sun's orbit and amassing a bunch of accreted rock and dirt, with the planet Earth eventually forming around him.
Now, since some 47 celestial bodies have aligned in such a way that the gravitational forces will disturb/awake Unicron, he's up and he's pissed. He's trapped and now has about 6 billion little "parasites" running around on his surface, which he's not happy about. Also, the only one who's shown up to help him is Megatron, who's not impressive enough to Unicron to even bother with. Optimus makes an impressive appeal to Unicron's vanity to spare the humans (he tries to make the big guy think that they're his offspring, a la Minicons), but Unicron's not having any of that, and just geopaths a bunch of rocky simulacra of his body (which, by the way, was once small enough to walk on Cybertron's surface) to fight Optimus.
Oh, I failed to mention that he's voiced by John Denethor/Dr. Bishop Noble! Awesome. However, I didn't think it sounded much like him, though I did think it sounded like a good Unicron.