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The Heavenly Ninja Party (忍天党, Ninten-tō) was a party composed of 21 graduates from the Ninja Village from all the different graduating classes. All of it's members were wiped out by the duo of Flashy Flash and Speed-o'-Sound Sonic.
The Heavenly Ninja Party was comprised of twenty one ninjas from the village. Despite the instructors of the village telling the ninjas that any kind of brotherly bond is prohibited, and that if they were to encounter each other post-graduation, they were to kill each other, the ninjas of different graduating classes decided to banded together, defying those previous orders.
The Heavenly Ninja Party was formed due to these ninjas sharing similar beliefs to form an elite assassination squad specializing in combat from generations of graduating classes.
Their main goal was to kill Flashy Flash, who they all hated due to him destroying the village; and to welcome "him" as their leader to publicly execute the aging Blast. Once this was done, they planed to go out of hiding and take over the world with their superior ninjutsu skills.
After the village was destroyed, many ninjas of the previous graduating classes decided to band together and form the Heavenly Ninja Party out of their shared beliefs and general hatred towards Flashy Flash for destroying the village.
After Sonic is once again defeated by Saitama and is recovering, he is approached by the Heavenly Ninja Party. This party consists of ninjas from previous graduating classes of the village, including Gale Wind and Hellfire Flame. The ninjas reveal to Sonic their plans to kill Flashy Flash and Blast, make the leader of the ninja village their leader, and come out of the shadows to take over the world with their superior ninjutsu skills. They tell Sonic to lure Flashy Flash out, where they'll ambush and kill him. They also warn him that those who betray them will be executed. Sonic, being outnumbered, has no choice and complies to their plan.
Sonic lures out Flashy Flash, and the two begin to battle. As they fight, Flashy Flash is impressed by how much stronger Sonic has become. Sonic states that he's not the same fighter from several years ago. Flash realizes this, and concludes that he has to use his ultimate moves to defeat him. It is at this point that the other ninjas attack Flashy Flash simultaneously with their weapons. The hero is able to dodge all the weapons, but is surprised by the number of people there and asks what this is all about. The fighters reveal themselves as ninjas from the village and their plan to kill Flashy Flash. Sonic then attacks one of the ninjas. He goes on to say that he could handle Flashy Flash, and for them to get out of his way. The ninja reveals his Ten Shadow Burial, impressing all of the ninjas watching him. The hero then decides to silence all the other ninjas, and their battle begins.
Flash and Sonic decide to work together to take down the group and succeed in slaughtering all the attackers.