Leon's Eternatus (Japanese: ゴウのムゲンダイナDande's Mugendina) is the fifth Pokémon that Gohcaught in the Galarregion, and his sixty-eighth overall. He gave it to Leon in JN127. In the past, it was known as the Darkest Day (Japanese: ブラックナイトBlack Night).
Ancient people feared Eternatus and it came to be known as the Darkest Day. It was later given its proper name by Rose as an indication of the infinite amounts of Galar particles it could produce.
Eternatus first appeared in Flash of the Titans!, where it flew past a plane that Ash and Goh were riding to Galar.
In Sword and Shield: "From Here to Eternatus!", Eternatus was revealed to have been captured by Macro Cosmos and placed inside the Energy Plant core. Rose planned to strengthen it with Galar particles so it could serve as a source of infinite energy for the Galar region. In order to complete his agenda, Rose sponsored Trainers and specifically groomed Leon to become a modern-day hero figure who would ultimately subdue and catch Eternatus to ensure it could be kept as a power source. After a number of experiments in which Galar particles were injected into the reactor, Eternatus escaped from its containment field within Hammerlocke Stadium.
Eternatus and Leon
Above the stadium, Eternatus began gathering massive amounts of energy from across the region, causing many Pokémon to Dynamax and Gigantamax and go on a rampage. Leon later tried to capture it with the help of his Charizard to seal away the Gigantic Pokémon but failed, and it subsequently turned into its Eternamax form. With the help of Zacian and Zamazenta, Ash and Goh defeated Eternatus, after which they threw one of Goh's Poké Balls at it together, catching it. After it was captured, the Dynamaxed and Gigantamaxed Pokémon returned to normal. Eternatus's Poké Ball was subsequently sealed away in a safe by Professor Magnolia.
Eternamax Eternatus reappeared in a flashback in Meeting Up with the Monarch!, where Leon revealed that being powerless against it helped motivate him to grow stronger.
In JN127, it was revealed that Eternatus had been unsealed by Professor Magnolia. Alongside Leon, the two had started to work to get Eternatus used to being around people. After reuniting with its Trainer, Goh agreed that was for the best and left it under Leon's care.
Personality and characteristics
Eternatus was feared by people, giving it the name of the Darkest Day. It is very powerful, easily defeating Galar's Champion and was only defeated by strong Trainers and other Legendary Pokémon. It is able to create and control Galar particles, which allow Pokémon to Dynamax. Through this, it can force or prevent a Pokémon from Dynamaxing.
Eternatus was dangerously aggressive, violently lashing at everyone around it, causing major havoc across the region. It is later revealed that it was periodically unsealed so it could get used to human contact, becoming far more docile, although Leon reveals that it lashed at him repeatedly. It is stated that Eternatus has difficulty controlling its power.
