While carving out villains of its own, the Boruto franchise is smartly playing to the strengths of old, bringing back certain enemies from the days of Naruto. A new terrorist group called Kara has emerged, similar to the Akatsuki that Naruto and his friends had to defeat. We're also seeing shades of Orochimaru (the reptilian scientist) emerge in Kara's leader, Jigen, as he blends science and the mystical elements of the ninja way.
In Boruto #30, however, a full-fledged villain makes an outright return to haunt Naruto's son, perhaps hinting the boy may be destined for a dark path, after all. This person is none other than Momoshiki Ōtsutsuki, a member of the godlike Ōtsutsuki clan, and to whom Naruto's literal life essence is tethered.
Momoshiki debuted in 2015's Boruto: Naruto the Movie, when we were first introduced to Naruto's immature son. The pale-skinned, blue-eyed villain came to Earth with his foster father, Kinshiki, to devour the planet's chakra. But it was more than just a random visit, as we'd learn in the Boruto anime that the Ōtsutsuki clan sought out God Trees across the cosmos, draining their chakra and rendering planets lifeless.
Kaguya (the main villain in Naruto's story) ended up betraying the Ōtsutsuki clan, using the God Tree to rule Earth, until Naruto and Sasuke stopped her. However, without her, they, as well as millions of ninjas, wouldn't have their power, so they do have to thank Kaguya for empowering their ancestors.
However, Momoshiki felt that these generations of chakra belonged to his people, so he came to feed, which ended with Boruto and his dad teaming up to slaughter him in a brutal finale. But in that fight, Momoshiki, just after death, appeared to Boruto in a vision, warning him this victory was the beginning of a dark path and that he would lose everything he loved in time.
At that point, many fans chalked it up to the final bits of Momoshiki's soul leaving the planet, with Boruto only being able to see it because of his Bakugyan eyes (the blue eyes he attained from his mother, Hinata, which was passed down from Kaguya). These eyes provide Boruto a unique vision where he can see chakra and certain spiritual aspects of the world.
The latest Boruto manga chapter confirms that Boruto's worst fears have come to life and Momoshiki is still haunting him. After sparring with his ninja rival, Kawaki (Boruto's own version of Sasuke), they give each other the Seal of Reconciliation (which is pseudo-handshake to show they're still friends), but as they touch each other, their special diamond-shaped mark known as Karma causes their powers to connect once again and Momoshiki manifests.
However, Boruto alone can see him as he continues to taunt Boruto and warn that death is coming to his hometown of Konoha. Boruto, weakened after the encounter, suspects Kawaki saw the villain too before he disappeared, but his rival opts to feign ignorance, apparently scared as well.
Naruto suspects something is off, though, as he seems to recognize Momoshiki's chakra. It's clearly linked to the boys' marks, which Koji Kashin, one of Kara's lead agents, believes is gained when a ninja defeats a member of the Ōtsutsuki clan. Naruto and Sasuke didn't get this mark, but Koji has hinted Jigen got his mark in the same manner as Boruto, and that's why he began experimenting on young boys -- so he could scientifically pass the mark on.
Of course, Jigen's only successful subject has been Kawaki, which is why his fate seems tethered to Boruto. The Karma mark makes them as powerful as Naruto and Sasuke were in their teenage years, and with Momoshiki returning for some mysterious purpose, it's clear they will need to unlock their potential as soon as possible. He appears to be back for revenge, and Boruto will need his entire family, as well as Kawaki, to topple what seems to be an eternal enemy once more.