Pokemon Zacian, Zamazenta, and Eternatus lore explained
The Pokemon Zacian, Zamazenta, and Eternatus are a crucial component to the history (and even the future, in some respects) of the Galar region. However, these mysterious creatures have plenty of backstory and lore connected to each of them, and their exploits in the Pocket Monsters core game series have helped shape the events and undertakings in Galar for generations.
With that in mind, it's worth examining what is known about all three of these creatures according to the Pokemon games (Sword and Shield) they appear in. Eternatus is quite different from Zacian and Zamazenta, but the antagonism between these Pocket Monsters is worth exploring due to the outsized impact it creates.
Breaking down the lore of Zacian, Zamazenta, and Eternatus in the Pokemon series
Zacian and Zamazenta, often referred to by fans as the Hero Duo or the Legendary Heroes, are believed to have been born beyond the Slumbering Weald eons ago. Many believe that the two Legendary Pokemon are siblings, or perhaps rivals who enact heroic deeds together and compete against each other. Whatever the case, these two creatures have existed in Galar since time immemorial.
The Legendary Heroes have regularly protected Galar from exterior threats, but don't always make their presence known, and they often slumber within the altar in the center of a lake in the Slumbering Weald. Arguably their greatest challenge arrived in the world of Pokemon 20,000 years before the modern timeline in the form of an extraterrestrial creature who crashed into and laid dormant in Galar.
This Pokemon was Eternatus, and its arrival in the world via asteroid seemingly created the Max Lair in Slippery Slope. The creature remained asleep for approximately 17,000 years. Roughly 3,000 years before the Sword and Shield game timeline, Eternatus awakened and began to siphon the energy of the Galar region, beginning an event in Galar's history known as the Darkest Day.
Eternatus' own energy resulted in the release of Dynamax/Gigantamax energy, creating Dyanamax and Gigantamax Pokemon. The supermassive Pocket Monsters went berserk and nearly tore apart Galar, but Zacian and Zamazenta awakened in their Crowned Sword/Shield forms to stop the calamity. With the assistance of two young trainers, the Legendary Heroes prevailed.
The two youths went on to become kings in ancient Galar, and Eternatus returned to its slumber. Parts of its body, imbued with Dynamax/Gigantamax power, rained from the sky as Wishing Pieces and Wishing Stars. However, as time and peace progressed, the people of Galar forgot about Zacian/Zamazenta and began to craft legends that the two youths or a nameless hero had saved Galar on their own.
For generations, peace reigned in Galar until the region encountered more turbulence during Sword and Shield's timeline. Due to an energy crisis taking place a thousand years in the future, Rose, president of the Macro Cosmos conglomerate, awakened Eternatus in hopes that he could tap into its seemingly infinite energy reserves, but the creature refused to be cowed.
Galar's champion trainer Leon attempted to stop Eternatus to no avail, leaving the task of defeating Eternatus down to a trainer canonically named Victor/Gloria (depending on the player's gender choice) and their friend Hop to succeed where Leon failed. They win, but Eternatus drains energy from Power Spots across Galar, transforms into its Eternamax form, and instigates a second Darkest Day.
Realizing they were outmatched, Victor/Gloria and Hop sought out the ancient items known as the Rusted Sword and Rusted Shield. The two legendary armaments awakened Zacian and Zamazenta to protect their home, engaging in a Max Raid Battle against Eternamax Eternatus. After a long-fought battle, Eternatus was defeated and caught by Victor/Gloria to prevent further calamities.
Since these events in Galar, Zacian, Zamazenta, and Eternatus have appeared elsewhere in the Pocket Monsters world, particularly in the Generation IX Scarlet and Violet games. However, these appearances have effectively no influence on Scarlet/Violet's overall story, and they're considered bonus encounters for trainers to enjoy as they explore Paldea, Kitakami, and Blueberry Academy.