The Ikari Clan
The Ikari Clan (ザ怒り一ザ) was a Race of Ninjas who were feared greatly in Battle during both the Second, and Third Shinobi Wars. Primarily because of their Kekkei Genkai known as Gekido.
They had been labelled as a bunch of mindless homicidal maniacs due to their Kekkei Genkai. Despite that Many countries had put tremendous amounts of Trust and Money into their Economy so they could gain powerful allies.
Unfortunately by the time Killer Bee was 15 years old, he had his seal tampered with by a group of Terrorist Ninja Extremists, in which his Eight Tailed form had unleashed a Menacing Ball upon the small sanction where the Ikari had resided.
Rei is the only Sole Survivor due to his kidnapping as a newborn infant by Orochimaru.
The Ikari's Kekkei Genkai is rather unusual, the effects of it are that they are able to Augment their chakra by immense scales of power in which they use to channel through their body to amplify their capabilities and senses.
The Gekido is Identified by the Users Eyes, their eyes turn black, only leaving the color of their eyes and pure black. It has been compared to the Cursed Seal's appearance.
As effective as it may sound, it is not without setbacks. The Chakra Augments the users Brain, the Power instilled cuts off their Frontal Lobes, Tampers with the Cerebrum, increases pressure on their Medulla Oblongata, and may sporadically affect the Central Nervous System on a very minimum scale.
What it means is that the Gekido deprives the user of being able to use Rational thoughts or Logic, Their memories are severely affected during the Gekido, They sporadically twitch and spasm due to the rush, and they Are extremely Aggressive, but the rush of Chakra fuels them on an Adrenaline high, Causing them to act like homicidal maniacs. Hence the saying "The Psychotic Laugh of the Reaper means the End".
Only very few members of the Ikari clan have been make the Gekido submit to their own will.