WARNING: The following contains spoilers for the Boruto: Naruto Next Generations anime.
In the Naruto franchise, we've seen quite a few forbidden rituals (or taboo jutsus, as the series often refers to them) that aren't for normal ninjas to practice. Most of them are attributed to Cursed Seals, or marks that are placed on individuals like hidden tattoos, turning them into walking terrors when activated.
These victims often get turned into instruments of mass destruction on par with powerful ninjas such as Naruto and Sasuke, but applying Cursed Seals has always been a long, complicated and tedious process, and the practice has become rarer since Naruto ended the Great Ninja War. In the latest Boruto: Naruto Next Generations episode, "The Cursed Forest," though, we find out the deadly application of Cursed Seals is still around, only this time it's deadlier than ever, having been weaponized like a drug.
Now, the history of Cursed Seals predates both the Boruto and Naruto sagas. In the past, they've been used to summon demons, but have also been repurposed to heal injuries for soldiers. Because the seals connect to nature's energy, they've predominantly been used to increase chakra in people and ramp up their powers exponentially. Sadly, this results in the individual going mad when the seal takes over -- think of Wolverine going into Berserker mode or Doctor Jekyll being controlled by Mister Hyde.
The results are catastrophic, as these individuals come off nothing short of demonic, thirsty for chaos and bloodshed. The most prominent example is when Orochimaru, one of the villains in the early Naruto era, applied one to turn Sasuke into a monster in his teenage years. It was eventually removed by Sasuke's brother, Itachi, but as we saw, that was no easy task. This is why, when Boruto's team and their senior, Konohamaru (Naruto's protege), stumble across an epidemic where seal marks are infecting a nearby settlement thanks to ducks being the carriers, they panic. Boruto, especially, knows the danger of the mark, as he was given a secret one he suppressed after battling Momokishi in his solo film.
This isn't the kind of epidemic Konoha (the Land of Fire) needs, as the virus is also spreading through the water supply, turning even more villagers into chakra-driven zombies. As the young members of the squad head from the outskirts back to Konoha's council to report what's happening, however, they encounter two mysterious teenagers with collars on their necks at the border known as the Land of Rivers.
Shockingly, these nefarious siblings, who appear to be robbers, active their devices and seal marks spread on them, turning them into superpowered villains. The heroes are shocked at what these implants can do because they know the repercussions of what they're encountering. They quickly surmise that armies of marked warriors can now be bred for war by something that's more or less a ninja steroid.
To make matters worse, these villains are in their senses, which means whoever is behind this has perfected the Cursed Seal technique and turned it into a piece of technology not even Orochimaru could have dreamt of back in the day.
With the villains' sanity intact and near unlimited power at their disposal, fans are eager to learn the identity of their puppet master, because when Naruto faced Sasuke with his seal, it almost killed him. As it stands, Naruto is in the dark, not realizing that an ambitious and apocalyptic scheme is right outside their doorsteps, intent on bringing war back to the Allied Shinobi Forces.