Carbuncle is an optional boss in Final Fantasy V encountered in Castle Exdeath. On the tenth floor, skull tiles cover the room. If the party steps onto the northernmost one, they will teleport to an inaccessible spot where Carbuncle is. There is a flight of stairs below if the party does not want to fight him. Defeating him wins Carbuncle, a Level 4 Summon. It is recommended to fight him, because after defeating Exdeath in the castle, the party will be warped to the Merged World and cannot come back to fight him again.
Carbuncle has the Auto-Reflect ability, so all magic cast against him will bounce back to the caster. During the fight, Carbuncle has two forms. The first form is the form with permanent Auto-Reflect, which involves Carbuncle using strong spells such as Blizzara and Thundara. The second form is Carbuncle's most vulnerable form, in which Carbuncle removes its Reflect and leaves it open to status attacks. During this form, Carbuncle will start using more dangerous spells such as Banish and Death. Carbuncle will only switch to the second form when its HP is depleted below half.
The player can bounce spells off of one of the party members with casting Reflect on them first. Otherwise, physical attacks do decent damage against Carbuncle, as does Gil Toss and Spellblade. In the Anthology, Advance, and iOS/Android versions, attacking Carbuncle with a Bio Spellblade will cause an instant win. Summoning Catoblepas will also defeat Carbuncle instantly. Alternately, the player just cast Reflect on one of the party members and bounce Petrify off them. The player can also use the Silence Spellblade so that Carbuncle never attacks.
Carbuncle was a mythical creature reportedly sighted in the Americas by Spanish conquistadors. It is described as a small creature, either a bird or a mammal, that has a gem in its forehead crystallized from the brain of a dead dragon. According to myth, it is good luck to catch a carbuncle. A carbuncle is also an archaic name given to any red cabochon cut gemstone. The name applied particularly to red garnet.