Divine Pulse (天刻の拍動 Ten Koku no Hakudō, Heavenly Pulse) is a game mechanic introduced in Fire Emblem: Three Houses.
During a bandit ambush by Kostas against several students from the Officers Academy, a mercenary group nearby lead by Jeralt stepped in to protect them. His child, Byleth, assisted as well, fending off most of the bandits. While Kostas was seemingly defeated, he suddenly sprung back up and promptly attacked Edelgard. Byleth leaped in between the two and was about to take a fatal blow before time suddenly froze.
In their deeper consciousness, they are greeted by a green-haired girl whom they had seen several times in a dream before, Sothis. Because of a spiritual link between the two, Sothis grants Byleth a limited ability to control the flow of time which she called Divine Pulse. Using its power, Byleth replayed Kostas's attack, this time disarming him and forcing him to flee.
Later in Chapter 2, Familiar Scenery, Sothis allows Byleth to retain her power to use during their journey, though she warns them that it has limited uses in battle.
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When several students go missing from the Officers Academy, Byleth, their students, and Jeralt investigates an abandoned chapel to find a few students cornered by monsters. After dispatching the monsters, revealed to be transformed students, a captured student, Monica, assassinates Jeralt when his guard is down. Byleth uses Divine Pulse to rewind time so they can stop her ploy, but Thales intercepts their attempt to stop Monica, leaving Jeralt's fate sealed.
Divine Pulse works similarly to Mila's Turnwheel from Fire Emblem Echoes: Shadows of Valentia, allowing players to rewind the course of a battle to correct costly mistakes. When activated, the player can reverse previous actions performed on a map up to a specific point in which they would like to restart from without having to restart from the beginning of the battle as a whole. These are rewound by individual actions of both Player and Enemy units. If Divine Pulse has at least one charge remaining and the battle's defeat condition occurs, Divine Pulse automatically activates to allow the player an opportunity to rewind turns immediately to a desired spot during the battle, thus preventing a Game Over unless all charges are used.
Byleth starts with three uses of Divine Pulse per battle and gains additional charges if the Saint Statues are restored by expending Renown, a certain Paralogue is cleared, and after clearing specific chapters. Byleth can have a maximum of 13 charges. In the Cindered Shadows DLC story, Byleth will always have a maximum of 5 charges.
You can use divine pulse by pressing ZL on your game controller.