"The Collision" was the name given to the one-day battle that resulted in the end of the universe.
In a timeline in which Emily Rutherford and Tom Charrington separated as teenagers, the Shandance and the Jatiri intervened in an interplanetary war. The ensuing battle, called the Collision, resulted in the end of the universe. When the Third Doctor encouraged Emily and Tom to stay together, the timeline was averted, and interplanetary war was avoided. (PROSE: Separation Day)