In Greek mythology, Polyphemus was an unfortunate cyclops who got on the wrong side of Odysseus, was deceived by the intrepid adventurer, and got a poker in the eye for his trouble. In God of War: Ascension, he's an enormous Titan who you get to beat upon with seven of your mates.
Here's the official blurb:
Known as the gigantic one-eyed son of Poseidon, the Titan cyclops Polyphemus is a brutal “boss” on our desert multiplayer map. He will literally crush you and your foes, countless times, until you earn enough favour from the Gods to be granted the Spear of Olympus – the only known weapon that can destroy Polyphemus.
There's a video of the beast below.
There's much more to be had over on the PS Blog.