"The Eighth Contract"
Kirei discovers Risei's body and informs Tokiomi. Kirei is saddened over his father's death, although Archer suggests that his sadness is due to not having the chance to kill his own father himself. Maiya informs Iri and Saber that Tokiomi wishes to form an alliance with them, and Iri agrees to go to the church to discuss the offer. Before the meeting, Tokiomi visits his wife and Rin, giving his daughter a magus book and some wisdom before leaving. During the meeting, Tokiomi suggests a temporary alliance to defeat the other contestants first to ensure outsiders don't attain the Grail. Iri agrees, but on the condition that Kirei leaves the city and can no longer be involved in the War. Tokiomi complies. As they depart, Iri reveals to Maiya that she is the embodiment of the Holy Grail in human form and will soon die to form the Grail near the end of the war. Maiya promises to protect her until that time comes. Meanwhile, Kirei reveals to Archer that his father's dying act was to leave Kirei the remaining Command Seals, restoring Kirei's ability to be a master. He also reveals the secret reason behind the war; the Holy Grail can be activated only by the sacrifice of seven Heroic Spirits. Thus, even a victorious Servant must die at the end, which is why Tokiomi is reluctant to use his command spells, as he needs at least one to command Archer's suicide. Furthermore, Kirei still wants to explore his dark desires, and Archer yearns for a more stimulating master. Later, Kirei meets with Tokiomi, who gives him guardianship of Rin should anything happen to him, and an Azoth dagger as a symbol of his completed apprenticeship. Kirei stabs Tokiomi in the back with the dagger, saying even his own father and master never really understood him. With Tokiomi dead, Archer and Kirei form a new contract as Master and Servant.
Source: http://en.wikipedia.org/wiki/List_of_Fate/zero_episodes