WARNING: The following contains spoilers for Boruto: Naruto Next Generations Episode 165, "The Quadruplets' Duty," now streaming on Crunchyroll.
The Boruto anime has given Team 7 a major obstacle in the form of the Haze Ninjas, poisonous shinobi who've stolen the Hashirama Cell and framed Naruto's son for killing Queen Sakuya. But one by one, these villains have all sacrificed themselves. After all, all the four siblings ever cared about was getting money for the weapon so they could enhance their impoverished homes.
And as Boruto finds out, these ninjas aren't any sort pushover at all, as they're driven by this desire to right the wrongs of the ninja alliance. Team 7 struggled to defeat Yūga after he turned into a Pikachu-like golem and now, as they face off against Hiruga, the heroes once more encounter a forbidden jutsu -- one that's literally as nasty as it gets.
Hiruga, sensing that Yūga failed and with Yoruga dying at Sakuya's home, leaves just him and Asaka. He instructs Asaka to get the DNA home and waits for Team 7. When they arrive, they fall into a trap as Hiruga has locked them down. He turns green just like his brothers do when they're enacting their special moves, unleashing his forbidden jutsu known as "Hellish Obliteration."
It creates a swamp filled with tar under the feet of Konoha's ninja, binding them tight. As they sink lower and lower, any move they shoot out is slapped away by tentacles rising from the inescapable muck. It's pretty disgusting but quite effective as it renders their chakra useless. The more they struggle or the closer they move to Hiruga, the faster they sink to their deaths -- like quicksand. There's absolutely nothing they can do about the sludge as it can only be stopped if Hiruga chooses to, or if he dies.
Boruto can't believe he's lived through so many battles at such a young age to die so easily, especially as they were all duped by Hiruga leaving water clones for them to attack. They operated like amateurs and are now paying the stinky price, with Hiruga revealing he'll be the gateman to take them to hell. He's lying, as a matter of fact, because as the ninjas discover, he's sinking faster than them and will die first. He even admits, as he nears his demise, that the technique is meant for stalling rather than combat.
It's a major downside but all Hiruga needs is to buy time -- he never intended to be an actual gateman to the underworld. This was all simply a dirty, mystical suicide mission, one in which his soul will be devoured in time but still enjoy knowing his brother's looking out for their people.
Admittedly, when he summoned the darkness, everyone expected the technique to be a bit more bloody and brutal, but with this move, once the shinobi survive, all they'll need is a bath. And luckily for Boruto's crew, Hiruga does have a change of heart as he realizes the maniacal Deepa is gunning for Asaka. So, he cancels the move to save his kin from certain doom while leaving the heroes desperately in need of a shower.
