Heavenly Ninja Party

The Heavenly Ninja Party (忍天党, Ninten-tō) was a group composed of twenty-one graduates from the Ninja Village, all of whom came from different classes, and were all eventually defeated by the combined efforts of Flashy Flash and Speed-o'-Sound Sonic.

They are the main antagonists in the Ninjas Arc.

Overview
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 planned to go out of hiding and take over the world with their superior ninjutsu skills.

History
After the village was destroyed, many ninjas of the previous graduating classes decided to band together and form an elite group due to their shared beliefs and general hatred towards Flashy Flash.

Ninjas Arc
Whilst recovering after being defeated by Saitama yet again, Sonic is approached by the Heavenly Ninja Party, which consists of ninjas from previous graduating classes of the Ninja Village; including Gale Wind and Hellfire Flame.

They explain their plan is to first get revenge on Flashy Flash and Blast by assassinating them, afterwards they'll convince the former village leader to join as the head of the party, and finally come out of the shadows to take over the world using their superior ninjutsu skills. The Heavenly Ninja Party explains they wish for Sonic to lure Flashy Flash out so they can ambush and kill him, warning him that anyone who betrays them will die.

Sonic, seeing as he's outnumbered, realizes he has no choice and complies to their plan. He proceeds to lure Flashy Flash to a secluded area where the pair begin to battle. During their fight, Flashy Flash says how impressed he is at how much stronger his junior has become to which Sonic explains he's not the same fighter from several years ago to which Flash agrees, concluding that he'll need to use his ultimate moves in order to defeat him.

It is at this exact point where the other ninjas, who've been waiting for the right opportunity, attack Flashy Flash all at once with their weapons.

While the S-Class Hero manages to dodge all their attack, Flash is still taken by surprised by the number of people there and asks what this is all about to which the Heavenly Ninja Party explains how they are from the village and wish to kill him for revenge. However, before they can proceed with their assault, Sonic attacks one of the ninjas and says that he can handle Flashy Flash on his own without their assistance. Flash and Sonic decide to work together to take down the group and slaughter all the attackers.

Before Gale Wind succumbs to his wounds, he tells them about the Ninja Village Leader, claiming that they'll never defeat him.