Warning: Spoilers ahead for Boruto: Naruto Next Generations Chapter 54
Momoshiki, Boruto: Naruto Next Generations manga's first-ever villain, reappears by possessing Boruto's body. In order to defeat the overpowered Kaguya Otsutsuki in the final arc of Naruto, Naruto and Sasuke, with the help of Sakura and Kakashi, have to give their all. Even with that, both of them still lose an arm. In Boruto: Naruto Next Generations, the manga sequel of Naruto, more Otsutsuki appear. Just like Kaguya, they are up to no good. Momoshiki eats Kinshiki to gain the power to fight both Naruto and Sasuke. However, he still ends up being killed by Boruto. At least that's what they've thought at the time.
Before perishing, Momoshiki manages to leave a mark on Boruto's arm. As the story progresses, it's revealed that the seal that grants him power during desperate times is actually called Karma and is a back-up file for Otsutsuki Clan members. In Kawaki Arc, Boruto meets another individual bearing Karma - Kawaki. However, he learns that even though they both bear the same symbol, they are vessels of different Otsutsukis. Kawaki is being prepared as Isshiki Otsutsuki, but he is eventually freed after they've killed Isshiki in Chapter 53.
Just when everyone thought everything is finally okay, Boruto suddenly attacks Sasuke. He has grown a horn and half his face is covered with Karma marks. Sasuke, Naruto, and Kawaki immediately identify that Momoshiki has completely taken control of Boruto's consciousness. They're forced to fight him despite the bad state they're in after the big fight with Isshiki, or Boruto will kill everyone except Kawaki.
Momoshiki explains that even though Kawaki's Karma has already vanished after Isshiki's death, around 80 percent of the Otsutsuki data has already been extracted in Kawaki's body, which makes him a viable sacrifice to the Ten-Tails. Sasuke figures out that Momoshiki's only able to come out after Boruto loses consciousness due to exhausting his chakra. Kawaki then uses ninjutsu to set himself ablaze, forcing Momoshiki to absorb the chakra since he can't allow his sacrifice to die. This little amount of chakra wakes Boruto, who forcefully breaks the horn to regain control of his body.
Momoshiki's appearance suggests that Boruto's body is ready to be possessed any time he loses consciousness. This means that Boruto has become a ticking time bomb. Momoshiki's revelation about Kawaki also signals impending doom. If Momoshiki indeed resurrects through Boruto and sacrifices Kawaki, then nobody will be able to stop him. However, fans can still stay calm. On the brief flashforward on Boruto: Naruto Next Generations' first chapter, both Kawaki and Boruto wields the power of Karma without any sign of being possessed. Perhaps shinobi has found a way to extract the Otsutsuki DNA from the Karma, turning it into a pure weapon, before Momoshiki takes over Boruto's body.