WARNING: The following contains major spoilers for Boruto: Naruto Next Generations #41 by Masashi Kishimoto, Mikio Ikemoto and Ukyo Kodachi, on sale now.
The Boruto manga has placed the next generation of Konoha's shinobi in the most dire of situations as Jigen wages war on Naruto's people. The villain wants to transcend into his final form as a member of the Ōtsutsuki clan, and he imprisoned the Hokage as part of that mission. But with Kawaki and Boruto using their Karma marks to teleport to the secret dimension in which Naruto is held, the situation has become even more complicated because of the evil bodyguard Boro.
He's as formidable as they come, but just when it seems as if he's going to take down the kids and bring them to Jigen, this generation's Naruto and Sasuke upgrade the Hokage's biggest weapon -- the Rasengan -- to show Boro they're far more dangerous than he assumed.
Boro is aware of just how lethal the two warriors can be due to the cursed seal known as Karma, but he's prepared for it, because Jigen knows everything about the mark. When he incapacitates this new Team 7 in the mysterious realm, he thinks he has the upper hand after poisoning the kids and weakening their chakra. However, because of Sasuke's daughter, Sarada, a window of opportunity is created, allowing Boruto to fly through the air and generate the ball of energy.
As he slams it into Boro's face, with his Karma mark activating, Boruto's Rasengan is larger and more powerful. The upgrade comes, though, when Kawaki latches onto Boruto, and his Karma mark actually gives Boruto a power boost. In other words, it's two Karma marks acting as the engine for the Rasengan. Using this magnitude of charka, it generates something on an atomic level known as the Unison. It's so devastating that it takes out the top half of Boro's body, incinerating his torso as Kawaki's energy makes the blast even larger.
It's the most brutal Rasengan ever, and one Naruto would be proud of. Not even he could have altered nature's energy to create something so destructive. We knew from his his first solo movie that Boruto had the ability to evolve it, and training with Konohamaru (to whom Naruto passed down the move) has found the Hokage's son leveling up his father's arsenal, big time. However, we never anticipated this kind of team-up changing the Rasengan so drastically.
That was passed from Jiraiya to Minato, Naruto's father, then to Naruto himself in the Shippuden era, and it's now being perfected by Boruto due to his Karma mark. And, as Naruto and Sasuke noted, with Karma, anything Boruto does is multiplied, and can wreak havoc on a level they could only imagine.
With Kawaki at his side, they are powered up exponentially, and it remains to be seen what original moves Kawaki will learn. He might even be preparing to adapt some of Boruto and Naruto's technique. However, one thing is for sure: If he does create some one-two combos with Boruto, it may form an alliance for which the ninja world isn't ready.