Time manipulation, or timeline alteration, is the concept of altering past events in such a way that it significantly affects the present or future. For example, in the classic Ray Bradbury story A Sound of Thunder, characters travel back in time to the prehistoric era. When one of the travelers accidentally kills a butterfly, the repercussions of that one butterfly's untimely death spirals into a chain reaction that has significant negative impacts on the present.
Examples in Video Games
Chrono Trigger's central plot element is the notion of time travel, and the player's actions in earlier eras can have significant impacts on what occurs in later periods of the timeline. As one example, in 600 A.D., there is a woman named Fiona tending to a desert with the dream of restoring it to life as a lush forest. If the player leaves Robo with her, he will assist her by working tirelessly to seed and tend to the soil. When the player next visits 1000 A.D., the once barren desert is now a lush forest, and Robo rejoins the party after his deactivated body is recovered from a shrine established in honor of both him and Fiona.
Valkyrie Profile 2: Silmeria
Unable to claim the valkyrie Lenneth in the present, the necromancer Lezard Valeth travels back in time to an era briefly visited in the original game. By enacting his plot over the course of Valkyrie Profile 2, he causes events in the timeline to play out differently, changing the futures of the three valkyries.
In the 2011 Mortal Kombat, Raiden receives a vision from his future self, moments before his death at the hands of Shao Kahn in Mortal Kombat: Armageddon. Raiden attempts to use this vision to alter events in a manner that will prevent the foreseen outcome, but in the process causes numerous changes to the timeline with significant, often negative impacts.
Fire Emblem: Awakening
In Fire Emblem: Awakening, it is revealed that the world is doomed to one day be destroyed by the fell dragon Grima, whose revival will lead to the world becoming overrun with undead creatures known as Risen. Chrom's daughter Lucina and a number of her friends and allies from the ruined future travel back in time in order to alter their past, and in the process forge a new future for the world.