Shinzon was a Romulan-made clone of Jean-Luc Picard who grew up among the Remans, seized control of the Romulan Empire and threatened Earth in 2379.
Shinzon was created by a Romulan faction as a clone of Jean-Luc Picard, meant to be swapped for the real captain as a spy. When a change of government ended the plan, the boy was sent to the dilithium mines of Remus, where he grew up among the enslaved Remans and became a hardened leader with the help of a Reman mentor, the Viceroy.
After distinguishing himself fighting in the Dominion War, Shinzon used a thalaron weapon to wipe out the Romulan Senate and made himself praetor. He invited the Enterprise-E to Romulus under the pretense of peace talks, having earlier used the android B-4 to lure the ship. Dying from the cellular breakdown caused by his cloning, he needed Picard's blood to survive and planned to use his warship Scimitar to destroy all life on Earth. Data stopped him by destroying the Scimitar's weapon, sacrificing himself in the process.
Shinzon serves as a dark mirror of Picard, a reflection on how circumstances shape character, and his scheme brought about the death of Data.