The Akatsuki
The Akatsuki are a band of rogue ninja organized for a specific mission. They have been ordered to collect all nine of the tailed beasts, although throughout most of the anime nobody knows why. It’s clear that each beast possesses enormous power, but the members of the Akatsuki aren’t after the spirits for themselves. Instead, they are collecting them to place in a vessel known as the Gedo Statue.
Of course, those that house the tailed beasts, known as the Jinchuriki, are no easy targets. These are well-trained ninja with tons of chakra who have been tasked by their nations to house what are essentially weapons of mass destruction. To stop them, the Akatsuki is made up of the strongest shinobi out there, some of which are quite legendary. There numbers include Sasori, a puppet master of the sands, and Itachi Uchiha, Sasuke’s own brother (to whom he vowed vengeance).
Their leader is unknown throughout most of the anime, although it is believed to be the masked man named Tobi. This individual has mysterious powers that allow him to teleport and completely avoid all attacks. He acts as a fool outwardly, but he is clearly very powerful. In fact, it was he who caused the destruction to Konoha Village that led to Naruto becoming a Jinchuriki and the Fourth Hokage’s death.