Einzbern Master

The unnamed Einzbern Master is the Master of Avenger in the Third Holy Grail War mentioned in Fate/hollow ataraxia and the Master of Ruler in the Third Holy Grail War of the Fate/Apocrypha universe.

Background
The Einzbern Master was a member of the Einzbern family. After their loss in the Second Holy Grail War, the family spent the sixty years of preparation time for the next war to claim absolute victory. They narrowed down all possibilities to win to two Heroic Spirits, Angra Mainyu and Amakusa Shirou Tokisada. They believed Angra Mainyu would be a living calamity able to slaughter all the opponents with ease and that Shirou's Command Spells as Ruler would allow for an extreme advantage. It could be said to have been a choice between "brains or brawn." Within the Fate/stay night universe, they summoned Avenger, and within the Fate/Apocrypha universe, they summoned Ruler.

In the Third War of the Fate/stay night universe, they quickly found out that their interpretation of Avenger was wrong, that he was a completely useless Servant without any abilities. They were defeated in only four days, and Avenger then corrupted the Greater Grail. After losing, it became common knowledge that they lost their place as a Master, all others avoiding them while they stayed in the Einzbern Castle.

In the Third War of the Fate/Apocrypha universe, they considered Ruler to be the safer choice of the two and decided to use caution due to their previous losses. Due to the fact that summoning a Ruler required great interference with the system, they had to summon someone who was only close to the status of a saint like Shirou. Although not outstanding in ability, he managed to continue to win and brought the Einzberns closer to the Grail than the previous wars. Due to he interference of Darnic Prestone Yggdmillennia and the Nazis making a bid for the Greater Grail, the war soon fell apart and the Einzbern Master was killed in the subsequent battles. Afterward, Ruler became incarnated and was formally adopted by Risei Kotomine

Fate/hollow ataraxia
The Master of Avenger's role is played by Illyasviel von Einzbern, taking the role of staying in the Einzbern Castle after having quickly lost Avenger after just four days.