By bonding with a Mirage Master, a Mirage can transform itself into a weapon and be wielded by said Master. The bond grants the Mirage Master a battle suit and various powers including magic, healing, and superhuman feats of strength and agility. By defeating lesser Mirages, they can collect materials that can be used to strengthen their Carnage, granting new weapons that in turn teach them skills or help to improve them.
Each Carnage has a selection of four skills associated with it, and the Mirage can learn these skills by fighting in battle and earning skill points upon victory, or escaping from battle after defeating at least one enemy before fleeing. Some of the stronger Carnages also can change the elemental affinities of their Mirage Masters.
During Chapter 3's Intermission, the ability to Class Change is unlocked. By promoting to a promoted class from their base class with a Master Seal, a rare form of Performa, a Mirage unlocks (typically) three new Carnages tied to that class. When a Mirage reclasses to their second class with an additional Master Seal, it unlocks the second set of promoted class-specific Carnages; despite popular opinion, reclassing does nothing else besides unlocking more Carnages with no further strings then the needing of a second Master Seal to reclass. In example, Chrom could first be promoted to Conqueror, which permanently unlocking Balmung, Tyrfing, and Laevateinn, then, once Chrom obtains the second Master Seal to reclass to Great Lord, it permanently unlocks Florete, Sol Katti, and Mystletainn.
During Chapter 4, the ability to reforge a Carnage that has had all of its skills learned is unlocked, allowing said Carnage to be powered up and its skills learned again. Upon being reforged for the first time, Carnages offer a fifth skill to learn in addition to its original four.