While it is not known to evolve into or from any other Pokémon, Zacian has a second form activated by giving it a Rusted Sword to hold. Its original form, Hero of Many Battles, will then become the dual-type Fairy/SteelCrowned Sword.
In its Hero of Many Battles form, Zacian is a large cyan lupine Pokémon with yellow eyes, a tan underside, a lean frame, a pink tail, and pink braids going down the side of its body. Two pink fringes of fur are present on its back. Battle scars are visible on its sides, and part of its left ear is missing. In its Crowned Sword form, the fringes of fur on its back become wing-like armor and its braids grow to resemble ribbons. Additionally, its head is covered by a golden helmet, and it carries a golden sword decorated to resemble Zacian's colors; when using the move Behemoth Blade, the armor on Zacian's back combines with the sword to increase its size and power. Zacian can sheathe its sword when not in use by wrapping it with its mane.
By absorbing metal particles, Zacian can change to its other form in battle. According to legends, Zacian was known as the Fairy King's Sword due to it being able to cut down anything in one strike, and it inspired awe in both its friends and foes. It is believed to be either the elder sister or rival of Zamazenta. It can turn itself into a statue in order to slumber until needed. It can cast illusions, including a projection of itself, even when asleep. It is also capable of flying. When together with Zamazenta, it can negate the power of Eternatus, even in its Eternamax state.
Zacian attacks with graceful, swift and captivating movements, which have been compared to a dance. It unleashes quick slashes that can't be seen by the naked eye; those struck by its sword will be cut down before they can feel the pain. The sword it holds in its mouth is really sharp and can cut through anything, even the thick scales of Dragon-type Pokémon as if they were just paper.
Moreover, it is tied with Mega Diancie for the highest base stat total of all Fairy-type Pokémon.
Crowned Sword Zacian is the heaviest Fairy-type Pokémon.
Despite being a gender unknown species, Zacian is referred to as Zamazenta's older sister in its English, Japanese, and ChinesePokédex entries; other-language versions use a gender-neutral word.
Zacian may be based on King Arthur's sword Excalibur. Its role as a warrior aiding a king may take inspiration on the Knights of the Round Table. In addition, its feminine nature may be a reference to the Lady of the Lake, the character responsible for giving King Arthur his sword, and its relationship with Zamazenta may allude to the relationship between King Arthur and his sister Morgan le Fay. It otherwise resembles a wolf, an animal once native to Great Britain and an important heraldic symbol for noble families during the Middle Ages. Its long, hanging blue fur resembles a caparison, a cloth covering usually worn by warhorses during the High Middle Ages.
The story of Zacian and Zamazenta retreating to the Slumbering Weald to recover from their wounds, and their return in the present day to save Galar, parallels the legend of King Arthur's messianic return.
Zacian's sword's ability to cut through anything, with Zamazenta's shield resisting any attack, could be an example of an irresistible force paradox. One of the most famous examples of this in folklore includes the Chinese tale of a peddler selling a spear and a shield with similar properties, only to be asked what would happen if the two items were to be used against one another. Another example tells of the Greek hunting hound Laelaps, which always caught its prey, being set upon Teumessian fox, which could never be caught, and had to be turned into constellations to prevent the paradox.
Collectively, Zacian and Zamazenta may also be based on Sirius, a binary star colloquially known as the "Dog Star". Specifically, Zacian may represent Sirius A, the brighter one. This star has a mass 1.1 times that of the Sun, which corresponds to Zacian's weight of 110 kg.
Additionally, the crescent shapes on its sword while using Behemoth Blade may reference Hati Hróðvitnisson, a wolf that chases after the Moon in Norse mythology.