Vendetta is a skill introduced in Fire Emblem Fates and appearing in Fire Emblem Echoes: Shadows of Valentia. In Fates, it is the personal skill of Rhajat.
Fire Emblem Fates
After Rhajat battles an enemy, she will deal an additional 4 damage when she triggers any subsequent battles with that unit.
Fire Emblem Echoes: Shadows of Valentia
In Shadows of Valentia, Vendetta adds half of user's accrued damage to his Attack stat.
The skill is activated when the user is holding Duma's Lance.