WARNING MAJOR SPOILERS for Star Trek X: Nemesis in here. This is everything I remember about him from the movie.

- Spoiler Space -

Played by Tom Hardy

Years ago, the Romulan military devised a plan to place a mole within the Federation, by cloning Captain Jean-Luc Picard and replacing him with a clone loyal to them. However, before they completed the project, a new government was put in place that scrapped the project. Shinzon, at the age of 9 years old, was sent to Remus to die in the dilithium mines there. While in the mines, he met a Reman who defended him against the guards, and eventually they escaped. On Remus, he built a warship, the Scimitar, and he has equipped it with technology banned in the Federation that generates a form of radiation with biogenic properties. It spreads out from its source, and turns all living things it touches to dust. After an ally of his activated the device in the Romulan senate, he ascended as the new Praetor and named the Reman who helped him in the mines as his new Viceroy. He sends a message to the Federation, claiming to desire peace, but his true intent is to capture Picard and then destroy Earth. When Shinzon was cloned, an plan was made to accelerate his aging to make him catch up with Picard, but it was not done and has caused his cellular structure to begin breaking down. He plans on a DNA transplant from Picard, but when he is unable, he attempts to destroy the USS Enterprise-E with his radiation weapon.

Source: Seing Star Trek: Nemesis tonight