Nago

Nago was the leader of a nearby Boar clan who fought off human advances from Iron Town as they sought to encroach on the mountainside. Initially, Nago met with success, and the humans were unable to establish operations or excavate the mountainside for iron ore due to Nago and the boars' attacks.

However, the tables were turned when Lady Eboshi and her riflemen arrived, putting a stop to Nago and the Boar clan's campaign. During battle, Nago was shot by Eboshi and left with a ball of iron within his flesh. The iron slowly began corrupting him, and when coupled with Nago's rage and fear of death, Nago became a Demon God and rampaged along the countryside until arriving after some time to Ashitaka's village.

During a heated battle in which Ashitaka attempted to plead with Nago for mercy on his Tribe, Ashitaka slayed Nago by firing arrows into his eye and skull. During the altercation, Nago cursed Ashitaka with the mark that would eventually see him expelled from his village.

Despite the Wise-Women of Ashitaka's village promising funeral rites for Nago, Nago died with hatred and regret in his heart. With his dying words, he promised the humans a similar fate to that which he and his boars had suffered.

Nago's death and transformation into a Demon God had a profound impact on the remainder of the Boar clan, who were seemingly now under the command of Lord Okkoto. The surviving boars were adamant to destroy Iron Town, and the loss of Nago's Boar Tribe at Iron Town starts the events in Princess Mononoke.

(Source: Ghibli Wiki)