In the years leading up to the Fourth Holy Grail War, no one could predict the terrors about to occur. One such disruption in the usual battle between Masters and Servants was the appearance of Fate/Zero's Caster. Accidently summoned by the serial killer Ryuunosuke Uryuu, who was merely attempting to summon a demon with the blood of a slaughtered family, the pair's only goals were to spread murder and terror through Fuyuki City. Their actions were significant enough to force the combatants to call a truce so that they could be dealt with. However, Caster's motivation was completely changed after a glimpse of his fellow Servant, Saber.
Using his crystal ball to peer into the first battle among Servants, Caster was filled with joy upon seeing Saber, declaring that his holy maiden had been returned to him. Finally finding a reason to leave his lair and the many victims that were slowly being tortured, Caster appeared before Saber and her master, Irisviel von Einzbern. As he began to celebrate their fateful reunion, Caster constantly referred to Saber as Jeanne d'Arc, or Joan of Arc, in his happiness. It became clear to all, except Caster, that this was a case of mistaken identity. In actuality, Saber was Arturia Pendragon, the true figure behind the legend of King Arthur, and not Jeanne d'Arc. Even when she tried to correct his mistake, Caster would not listen, determined to make Jeanne his own.
Caster's desire originated from his previous life and former identity: Gilles de Rais. As a French nobleman in the 15th century, Rais was bound to fight in the Hundred Years War. He would come to serve under Jeanne d'Arc, who became the object of his admiration and love. Unfortunately, when Jeanne was burned at the stake as a heretic, this heroic woman's cruel fate drove him into insanity. He became a prolific serial killer, targeting the children of his lands. After years of madness, Rais would be executed by his fellow nobles, not as punishment for his crimes but as part of a plan to steal his lands. At the end of his life, Rais had an estimated 140 victims and became the inspiration for Bluebeard, another name used in the Fourth Holy Grail War.
Beyond appearances, there are many similarities between Saber and Jeanne d'Arc. These warriors were forced to hide their true identities during their lives, conforming to the gender roles at the time. Both also met tragic fates, being betrayed by their own countries for fighting for what they believed to be right. Seeing the stories of these women, or one woman in Caster's mind, only fueled his worldview that all of them were at the mercy of a cruel God whose punishments hardly ever match the crime.
After realizing that Saber didn't know him, Caster, in his madness, came to the conclusion that God had removed Jeanne's memories and gave her a false cursed life as his punishment. This led Caster to focus on the number of his victims, kidnapping more children and sacrificing them so that he could amass enough power to free "Jeanne" and return her memories. Amid this goal, Caster also repeatedly tried to convince Saber to abandon her righteous ways, something other Servants did as well. He argued that only cruel actions belonged in such a world. While he had planned on only having fun in the modern world, this obsession forced Caster to take bigger risks and to actually battle his fellow Servants.
After failing to defeat Saber and turn her to his own dark path, Caster's rage against God was shifted by his master's own demented philosophy. Deciding to salute their cruel world, the two unleashed a Kraken-like creature upon the city. This final battle brought all the Servants together, either working with one another or still battling among themselves. Weakening the creature enough, and Caster who resided within it, Saber would deliver the final blow using the holy sword Excalibur and her Noble Phantasm. Before his death, Caster would come to realize that Saber was not his beloved Jeanne, and ultimately regretted the path of sadism and cruelty he had taken in light of her death.