The mysterious figure is indeed Unicron.
Do remember that Unicron is neither Autobot nor Decepticon, hence the blue eyes.
And given his origin of creation, he's only there for one thing: multiversal domination.
How is he going to start? By eliminating Prime.

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Added on December 12, 2010, 11:59 amI also strongly believe that the movie title fits Unicron's description.
Remember, Unicron can transform into a moon.
And as Calvin871989 had explained, it also means the "alien" landing on the dark side of the moon.

This post has been edited by AzrinRain: Dec 12 2010, 12:08 PM

@absorb-d,
yes, i do agree on you with the size issue. Unicron indeed has a size relative to that of the moon(may be bigger). I am inclined to say it might be Alpha Trion, but this guy (let's call him Robot Gandalf for now), but the details look more matching to Unicron.

This post has been edited by AzrinRain: Dec 12 2010, 05:00 PM

Alphacron? If that happens, Bay is gonna kill Transformers, man.

JonSpark,
if it really was Optimus, then it will not help to explain his arrival in Transformers (first movie)