Caster of Black, whose true name is Avicebron, also known as the 12th-century poet and philosopher Solomon ibn Gabirol, is a Servant summoned by the Yggdmillennia faction in the Great Holy Grail War. Born in Málaga, Spain, he was not a warrior or a king, but a thinker and magus whose work on the Kabbalah would heavily influence both the history of thaumaturgy and the European Renaissance. In life, he suffered from poor health, particularly a serious skin disease, which contributed to his reclusive and pessimistic disposition.

Avicebron is an eccentric and intensely focused individual who rarely expresses emotion, communicating largely through silent nods. He wears a featureless mask and a full-body suit, making his feelings impossible to read. His sole obsession in the Holy Grail War is the completion of his ultimate Noble Phantasm, Golem Keter Malkuth, a giant automaton he envisions as a paradise-creating mechanism that he was unable to finish during his lifetime. This singular goal defines his entire personality, as he is willing to trample anyone underfoot to realize his dream.

He is summoned by a young and gifted magus named Roche Frain Yggdmillennia, who becomes his Master. Roche is a prodigy in golem craftsmanship and holds Avicebron in the highest esteem, referring to him as "Sensei". Avicebron is impressed by the boy's talent and passion, recognizing that in another era, Roche could have been his ideal pupil. However, their relationship is ultimately a tragic one, as Avicebron sees the boy's outstanding magical circuits as the perfect core material for his incomplete Noble Phantasm.

In the early stages of the war, Avicebron spends most of his time within his workshop in the Yggdmillennia fortress, manufacturing golems to serve as the faction's main fighting force. His creations are durable enough to withstand attacks from powerful Saber-class Servants and are also used for reconnaissance and scrying. At one point, he successfully captures Spartacus, the Berserker of Red, and becomes his proxy Master, using the rebellious Servant as a weapon against the opposing faction.

Avicebron's defining act of betrayal occurs during a confrontation with the Red faction. When offered a chance to join Shirou Kotomine, he immediately betrays the Black faction on the condition that his original Master, Roche, be spared. After forming a new contract, his first act is to seize Roche to use as the living core for Golem Keter Malkuth. The completed golem, a colossal giant that brings plant life to barren land with its footsteps, is unleashed upon his former allies. Avicebron shows no hesitation or regret in this act, demonstrating that his long-held dream completely overrides any loyalty or affection he may have felt for his student.

His story concludes within the giant golem itself. As the massive construct fights against the combined forces of the Black faction, Avicebron merges with it completely, sacrificing himself to power his masterpiece, believing it will finally create a paradise. In later appearances, such as in Fate/Grand Order, Avicebron explicitly recalls this act of murdering his young Master. He treats the memory as a sin, willingly sacrificing himself in atonement for his past betrayal, showing that his character is haunted by the consequences of his obsession.