The Calcabrina, also known as Calcobrena, Calbrina, and Calbrena, is a boss in Final Fantasy IV. It appears during the battle with the Calcas and the Brinas after a certain amount of time passes and the enemies are not defeated. If this happens, the Calcas and the Brinas will use Combine, and the party will have to battle the Calcabrina.
Calcabrina can be dangerous because it uses Glance to charm a character and can cause large amounts of damage. If the Calcabrina is not defeated within a certain amount of time, it will revert to the Calcas and the Brinas with all their HP.
If the party is at a high enough level, Calcabrina should not pose much of a threat, and there is also a simple method to taking it down without it transforming.
The party should take out one Calca, then one Brina, and repeat, using Cecil to attack the Brinas and serve as back-up healer, Kain using Jump on the Calcas and Yang attacking the Calcas as well; Rosa should serve as primary healer and attack with Aim when possible. Eventually, when only one Calca and one Brina are left, Kain should use Jump on the last Calca, and once it is gone, the party should quickly take out the Brina. The dolls transform into Calcabrina when all of the Calcas or all of the Brinas are active without having taken out all of the other group as well. By taking out a group, the player will only have to deal with one enemy (this case, Brina, which has less HP), so if done correctly, it will perish before being able to transform.