Thalaron radiation is a rare and dangerous form of energy that destroys organic matter, used as a weapon by Shinzon of Remus aboard the Scimitar in 2379.
Romulan scientists discovered thalaron radiation but abandoned research on it because of its extreme hazards. Even small amounts can kill all organic life it touches, leaving no survivors over a wide area.
Shinzon, a clone of Jean-Luc Picard raised among the Remans, revived the research. He used a thalaron device to kill the Romulan Senate in a coup that made him Praetor, then built the radiation into the Scimitar, a massive warship whose thalaron weapon could sterilize entire planets. He intended to turn it against Earth.
The Enterprise-E fought the Scimitar in the Bassen Rift as the weapon charged. Data sacrificed himself to stop it, transporting Picard back to the Enterprise and destroying the Scimitar's thalaron generator with a phaser shot, killing Shinzon and himself.