Phoenix Mode is a mode introduced and currently exclusive to Fire Emblem Fates.
Phoenix Mode is an easier mode for Fire Emblem Fates designed to allow the player to move through the story without having the gameplay hinder their story progress. Phoenix Mode functions similarly to Casual Mode where the Permanent Death feature of Classic Mode is disabled. However, where Casual Mode still removes a unit who has lost all of their HP for the rest of the battle, Phoenix Mode merely incapacitates them until the start of the next Player Turn, where they are revived at full health. Enemy units are also unable to utilize Attack Stance and are only able to negate dual attacks in Guard Stance.
As the lowest Mode setting, it cannot be changed back to Classic mode if lowered from it originally, but it can be changed back to Casual mode at any time. If lowered from Classic or Casual on Hard or Lunatic, the difficulty will also automatically lower to Normal if this option is selected.