The destruction of the Federation colony on Delta Rana IV and its eleven thousand inhabitants by the Husnock, answered by a grieving Douwd who wiped out the entire Husnock species.
In The Next Generation episode The Survivors, the Enterprise-D answers a distress call and finds Delta Rana IV scoured bare except for one patch of garden and a house. There live an elderly couple, Kevin and Rishon Uxbridge, who cannot explain why they alone were spared and refuse to leave.
A huge warship appears and vanishes by turns, and Deanna Troi is tormented by music playing ceaselessly in her mind. Picard concludes that the ship and even Rishon are illusions and that Kevin is their source.
Kevin admits he is a Douwd, an immortal being who lived as a human for love of Rishon. A pacifist, he would not fight when the Husnock attacked, and she died defending the colony. In an instant of rage he then killed every Husnock everywhere, fifty billion in all. Picard, unable to judge such a crime, leaves him to his solitude.