Fatigue is a gameplay element introduced in in Fire Emblem: Thracia 776.
As units perform actions in the game, such as fighting, dancing, and using staves, a number accumulates on the second page of their status screen. This is their fatigue meter, and if their fatigue meter exceeds their maximum HP, they will be unable to take part in the next chapter. A unit's fatigue meter is reset to 0 after any chapter in which they are not deployed, regardless of what it was before. Alternatively, a rare Stamina Drink can be placed in a unit's inventory when on the Preparation Screen to reset their fatigue. Leif, as the main Lord, is immune to Fatigue as he is required in every chapter.
Usually, a unit's fatigue meter is only increased by 1 for each time they enter combat, but high-level staves increase fatigue by greater amounts; D-rank staves increase fatigue by 2, C-rank staves increase fatigue by 3, B-rank staves increase fatigue by 4, and A-rank and Preferential staves increase fatigue by 5. This can be problematic, as most staff-wielders have a low Max HP.
Fatigue caps at 99, although it is unlikely to meet this cap, excluding for Chapter 8, when all the fatigue from the Manster Arc is taken into consideration, unless the player is engaging in some form of EXP-grinding, such as Arena Abuse or Boss Abuse.
Shadows of Valentia
Fatigue works similarly in Fire Emblem Echoes: Shadows of Valentia, although to a noticeably less severe extent. As a battle occurs, player units will slowly accumulate fatigue. Each character has a different Fatigue Tolerance that determines how quickly they're Fatigue. Entering combat, taking damage in combat, taking damage from terrain, and using healing spells build fatigue points. Once the character's accumulated Fatigue points equal their Maximum HP, upon the next battle, they will suffer an HP reduction instead of Thracia's forcible benching. The HP reduction depends on the character's Fatigue tolerance. Hence a 30 HP Fatigued Celica will have 18 HP instead. Unlike Thracia 776, all units can experience Fatigue, including Alm and Celica.
Fatigue only has a noticeable effect when entering successive battles within the game's dungeons. Upon exiting a dungeon, all fatigue is removed. Fatigue also does not carry over from battles fought on the world map. Players can reduce fatigue by feeding the unit a provision. Each provision restores a certain amount of Fatigue depending on the character's taste preferences. Additionally, offering a provision to a Mila Idol will refresh all members of the player's party, removing all fatigue from every unit present.