Polyphemos is a Cyclops. The player does not encounter another Cyclops until Mount Olympos. Polyphemos is encountered on the Halcyon Coast.
Polyphemos has several abilities. One is his stunning scream. It slows you down considerably, and disrupts your skills for a few seconds. The scream also takes away 50% of your health, no matter what your level is. Another is his stomp. It does quite a bit of damage and stuns you for a few seconds. Polyphemos has quite a bit of health and has a strong default weapon attack.
Polyphemos is a very difficult boss for unprepared players. You get a small legion of Spartan soldiers to help you, but they are very weak. Let Polyphemos kill them, or they can steal the experience from you if they get the last shot. If you can get stun or skill disruption protection, this fight will be much easier. His scream is very slow, so if you're moving it's very easy to avoid it.