After defeating Eternatus, Eternatus transforms into Eternamax Eternatus. When Hop teams up with the player to confront it, no Pokémon can successfully use a move for the first three turns of the battle. Then, the player and Hop use their Rusted Sword and Rusted Shield items to summon Zacian and Zamazenta, which will team up with them against Eternamax Eternatus. The battle is similar in style to a Max Raid Battle (e.g., no Poké Ball may be used during the battle), but features some key differences:
Eternatus uses only one move per turn, which is always a Max Move.
The Tower Summit appeared in a series of flashbacks in The Champion as Leon recalled the events of the Darkest Day. He had battled against Eternatus, but failed to catch and stop it. Due to him being caught within the blast of Eternatus escaping from his Poké Ball, he was unable to continue fighting, leaving it up to Victor and Hop to continue the battle. Just when it seemed like Eternatus was defeated, however, it proceeded to transform into its Eternamax form. Using the Rusted Sword and Rusted Shield, Victor and Hop were able to call Zacian and Zamazenta to help them. With their aid, they were able to weaken Eternatus enough for Victor to catch it and thus put a stop to the Darkest Day.
The Energy Plant first appeared in PASS17, where Raihan infiltrated it to find out what the explosion that had happened there was all about. He was soon found by Chairman Rose and Oleana, who explained the situation away as a failed experiment. Rose explained that there would be an energy crisis one thousand years from now, and they were attempting to develop a new energy source for Galar out of Wishing Stars. Raihan was unconcerned, given the long time before anything would go wrong, and told Rose to consider publicly asking for help instead of hiding his experiments. After leaving, Raihan mused his suspicions about the way how Rose had placed Wishing Stars in a bowl, as if as an offering to something.
In PASS26, after Chairman Rose had started the Darkest Day, most of Galar's Gym Leaders, Henry, Casey, Hop, Marnie, Bede, Leon, and Sonia entered the Energy Plant to follow Sordward and Shielbert, who had taken the elevator down there a bit earlier. At the heart of the Energy Plant, they encountered Rose, who declared that his experiment to develop a source of infinite Galar particles had been a perfect success. The others disagreed, but Rose showed them that Macro Cosmos was already taking action in keeping the rampaging Dynamax Pokémon under control, before revealing that he had only been present via a hologram and locked everyone in to keep them from interfering.
Meanwhile, on the Tower Summit, Sordward and Shielbert started battling against Eternatus. When Rose was informed that Eternatus wasn't producing enough Galar particles yet, he urged the two brothers to drive it further. They took out the Rusted Sword and Rusted Shield they had stolen from the Slumbering Weald a year earlier, but while Eternatus flinched at the sight of them, they didn't transform and manifest their power in the way the brothers had expected. When Sordward and Shielbert were subsequently almost sent flying off the tower, Rose decided to take matters into his own hands and challenged Eternatus himself, using his GigantamaxCopperajah to face the Gigantic Pokémon.
At the same time, Oleana entered the Energy Plant and requested everyone present to lend a hand, as she didn't want to see Rose losing his life in the battle. Just as Rose was defeated, the Gym Leaders and their allies arrived at the Tower Summit and started battling against Eternatus. While the battle raged on, Casey's Falinks and Stunfisk, whom Sordward and Shielbert had also stolen a year earlier and lost moments earlier when Eternatus had sent them over the tower's edge, returned to Casey, along with the Rusted Sword and Rusted Shield. Eventually, the Gym Leaders succeeded in defeating Eternatus, only for it to turn into its Eternamax form and easily defeat them. Thankfully, Zacian and Zamazenta then arrived and started fighting against Eternatus.
Meanwhile, Henry, to his horror, found himself unable to restore the Rusted Sword and Rusted Shield to their rustless states and refused to give them to Zacian and Zamazenta. He stubbornly kept holding onto the artifacts even as they were drawn towards Eternatus. When the Hero duo attacked Eternatus in PASS30, they, Eternatus, Henry, and the artifacts disappeared in a bright flash of light. While the injured ones were being treated, Casey reviewed the Drone Rotom footage of the battle and discovered that Henry and the artifacts had disappeared moments before Eternatus, leading to her and Marvin figuring out that they had been teleported away by an unseen Pokémon.