In the context of Fire Emblem, this event always occurs upon the death of any Lord character in the game or an Avatar, as the game's plot revolves around their story, thus the plot cannot continue with their death.
Some maps throughout the series require that the player either prevent an enemy from killing an NPC, escape a map/reach a certain point in the map in a set number of turns, or preventing the enemy from reaching a specific point on the map. Failing to fulfill these requirements will result in an instant Game Over. Other situations that trigger an instant game over include having the enemy subdue the player's base of operations in Fire Emblem: Genealogy of the Holy War, attacking or killing Fargus in Chapter 16x of Fire Emblem: The Blazing Blade, allowing a Begnion Dragoknight to kidnap Leanne and successfully escape with her in tow in Part 2's Prologue of Fire Emblem: Radiant Dawn, or allowing an enemy to pass the green line in Part 3's Chapter 13 in Fire Emblem: Radiant Dawn.
In Fire Emblem: The Sacred Stones, in skirmishes, Tower of Valni, and Lagdou Ruins, if the player didn't bring Eirika or Ephraim into the battle, and all the characters the player did bring die, a Game Over still occurs, despite the fact that Eirika or Ephraim didn't die. This likewise occurs in Fire Emblem Fates under the same circumstances, even assuming that Corrin is absent.
An instant Game Over in TearRing Saga: Utna Heroes Saga, aside from the characters who are required to defeat the final boss's death, will occur if the player hasn't defeated the final boss before Turn 20: Gerxel will cast Black Rain, killing everyone except Tia, who Gerxel intending to gloat at her subsequent despair before absorbing her life energy and regaining the full extent of his heinous powers.
In Fire Emblem Heroes, despite there's no permanent death in the game, there are a lots of Game Over that can occur such as, in Grand Hero Battles and Bound Hero Battles, if a unit is defeated, if the turn limit has passed and the player loses all their hearts in Tap Battle. Initially, getting a Game Over forfeited all EXP or SP from the battle, but later versions changed that so that EXP and SP are retained. In modes where they are not disabled, it's possible to use Light's Blessing to recover from a Game Over, with the exception being turn limit Game Over.
In Fire Emblem Warriors, all chapters are time-limited. Usually the time limit is 1 hour, but sometimes (usually in History Mode battles, but also in main story Chapter 7) it is shortened. If the time runs out, the Game Over will occur.
In Fire Emblem Three Houses if a map's Defeat condition is triggered, the Divine Pulse mechanic immediately activates so long as there is at least one charge remaining and the player can immediately rewind time to the appropriate action they wish to restart from without having to completely restart from the beginning.
If a Game Over occurs, players must restart that chapter or restart from the last save point.