The Betazoid Massacre is one of the worst atrocities of the Galactic Empire. The masssacres were carried out by Grand Moff Willhuff Tarkin in 5 BBY.
The world Betazoid was one of the major planets in the Star Trek dimension. It was well-respected by many across the Universe, and had been a close ally of the Jedi Order. It's native species, the Betazoids, were almost human, except they all had dark black eyes. They also had telepathic detection abilities. They were able to detect the motives of others, to read the minds of others, and to detect their thoughts. Half-Betazoids could only detect other's emotions. For these reasons, the Betazoids were considered a threat by the Galactic Empire. Emperor Palpatine feared they would try to use their telepathic powers to challenge his rule. So in 5 BBY, he authorized Willhuff Tarkin, Grand Moff of Scientific Oversector, which included the Star Wars and Star Trek dimensions, amongst other science-fiction dimensions, to conduct massacres of the Betazoids. Tarkin was a close associate of the Emperor, supervisor of the Death Star construction project (and eventually it's supreme commander), and a powerful military and political official. Tarkin was also one of the most ruthless individuals in the Empire, responsible for the Ghorman Massacre, the Cleansing of the Animated Dimensions, the enslavement of numerous alien species, including the conquest and enslavement of Kashyyyk and Mon Calamari and the enslavement of Qu'onos and Romulus, and numerous more atrocities.
Under the orders of Grand Moff Tarkin, Imperial spies were able to shut off Betazoid's shield generators before being detected and detained by the Betazoids. Shortly after this, an Imperial fleet of 38 Star Destroyers appeared over the world. Tarkin demanded the Betazoids surrender, but they, detecting his and the Imperials' motives, refused. Shortly after, Tarkin ordered a precise bombardment of the world to be conducted. The Star Destroyers unleashed a Base Delta Zero. All Betazoid cities and settlements were destroyed, and the planet itself was left a desolate wasteland.
More than 28 billion Betazoids had died in the Massacre. This was a relatively large amount, even larger in the amount who would die in the Destruction of Alderaan in 0 BBY. Tarkn's reputation as "the Imperial Angel of Death" was even more consolidated. Fear increased across the Universe, but outrage did as well. The Empire however would use the massacre to cow more overt opponents into submission.