Combat Arts (戦技 Sengi) or simply Arts are a gameplay mechanic introduced in Fire Emblem Echoes: Shadows of Valentia.
In Shadows of Valentia, Combat Arts are special skills and techniques that characters can use in battle. Unlike Skills which are learned based on a character's current class, Arts are learned by using specific weapons, shields, or rings. The more a character uses an equipment item, they begin to learn new attacks with different properties from the standard attacks. They can increase range, deal effective damage against a specific type of enemy, or deal more damage. Once a character has learned an Art, any subsequent weapon that has the same Art as the ones a character has learned will be automatically usable from the start. To activate Combat-type Combat Arts, a character must first sacrifice a set amount of health.
In Three Houses, Combat Arts are gained by sharpening a character's skills in their training, be it from honing their skills in weaponry, special unit classes, mastering a class they have unlocked in their Certification exams, or unlocking a Budding Talent by being tutored. Like the Valentian Arts before, the Fódlan Combat Arts can have different properties from standard attacks, and can vary in purpose, be it from being a simply stronger attack or be supporting in nature, such as moving ally units, or healing oneself. Using weapon-type Combat Arts, however, weakens a weapon's Durability faster than normal. While characters are mainly limited to having only 3 Combat Arts equipped at once, certain characters that have Crests can use a fourth, personal Combat Art when they wield a weapon-type Hero's Relic that corresponds to their Crest. In addition certain classes will enable a special Combat Art if you have mastered the class and are currently in said class, this Combat Art is automatically equipped and does not count against your three slots either.
Dimitri has the Crest of Blaiddyd, which allows him to occasionally double both the Might and Durability costs of his Combat Arts when it activates. Lysithea and Catherine have the Crest of Charon, raising the Mt of their Combat Arts when it activates, which is also the case for Sylvain and his Crest of Gautier, Ingrid and her Crest of Daphnel, and characters that possess the Crest of Seiros, those being Rhea, Jeralt, and Edelgard. Claude has the Crest of Riegan, which allows him to occasionally recover about 30% of his health when using Combat Arts, which is also the effect of the long lost Crest of Chevalier. Seteth and Ferdinand have the Crest of Cichol, which allows them to occasionally stop enemies from performing counterattacks with their Combat Arts, which is also the case for Hilda and her Crest of Goneril.