Curse of Hatred

The, is an ideal of enmity that was said to have been passed down throughout the generations by the Uchiha clan's forefather to the members of his lineage. The origins of this curse can be traced back to when a dying Sage of the Six Paths took the fateful decision to name his younger son, who would go on to become the forefather of the Senju clan, his successor instead of the older sibling because the Sage believed that the beliefs held by the younger son were the key to peace. Consumed with hatred, the elder brother eventually waged war against his brother and thus began a cycle of hatred that transcended history, culminating in the mutual hostility between the Uchiha and Senju.

As Tobirama Senju revealed, the Uchiha treasured love and friendship even more than their own clan, yet it was their ultimate downfall. Because this is a part of them they regularly conceal, an Uchiha's love turns into a stronger hatred the moment they lose the ones they hold dear. This despair-filled experience would cause a manifestation of special chakra within the Uchiha's brain, affecting the optic nerves and creating the Sharingan, "the eye that reflects feelings". Even the ways for Sharingan to evolve further into Mangekyō Sharingan and Eternal Mangekyō Sharingan require extremely painful experience that would drive the user into an even deeper darkness that would utterly consume them. Such a ninja at the time was Madara Uchiha, who refused to relinquish his hatred ever after being forced to take his beloved brother's eyes to obtain the Eternal Mangekyō Sharingan prior to Konoha's founding. Infuriated that his rival Hashirama Senju was made First Hokage instead of himself, Madara sowed the seeds of contempt among his kin with the promise that they would eventually lose standing in their partnership with the Senju if they did not help him.

Though Madara was exiled by his clan soon after, a few Uchiha would begin to believe his warning generations later, yet they were no longer in a position of power to do anything about it due to Tobirama's attempt to thwart the curse by placing the Uchiha in the Konoha Military Police Force to regulate their emotions. The Uchiha's curse was fully rekindled later as the result of Obito Uchiha witnessing the death of Rin Nohara, in turn causing the Nine-Tails' Attack on Konoha which resulted in the Uchiha plotting to take over Konoha after being unjustly accused for the event. This led to the Uchiha Clan Massacre executed by Itachi Uchiha, with Sasuke Uchiha's quest for avenging his family resorting in becoming what Obito noted as the sole inheritor of the Uchiha Clan's hatred and thus fated to clash against Naruto Uzumaki, who has inherited the Senju's Will of Fire. However, questioning his resolve, Sasuke has Orochimaru reincarnate the Hokage and learns the full nature of the Curse of Hatred from Tobirama.

Trivia

 * Although this is said to be the ideal of the members of the Uchiha clan, two known members, namely Itachi and Shisui's ideals were the opposite of this, and seemed to follow more closely to the ideals of the Will of Fire.
 * As stated by Tobirama Senju, this Curse of Hatred is a rival of Will of Fire.
 * It was said that; if they lose their loved ones, or something bad hapenned to them, it will activate their Curse of Hatred, thus, unlocking the Sharingan.