Giulio d'Este (1478–1561) was the illegitimate son of Ercole I d'Este, Duke of Ferrara.
Giulio d'Este and Cardinal Ippolito, both of them brothers of Alfonso I, held grudges and differences with each other over the course of their lives. In 1505, their father, Ercole I, died on January 2. Following this, Ippolito snatched away a prized musician, (the bragging rights of the 16th century) Don Rainaldo, from their father. Allegedly, Giulio later 'stole' this singer from the castle where Ippolito was holding him. Using his influence over Alfonso, Ippolito ensured that Giulio was exiled from the immediate Ferrara area and put under house arrest.
Later the two both fell in love with the same woman, Angela Borgia, cousin of the Duchess Lucrezia. This gracious and much-courted maiden granted her preference to Giulio; one day, to attested to Ippolito that "Monsignore, your brother's [Giulio's] eyes are worth more than the whole of your person..." If mad before, Ippolito became insatiably furious; he planned an ambush.
Giulio was alone and could do nothing when the Cardinal ordered his soldiers to seize, kill, and stab Giulio. The order was partially carried out, as Giulio brutally beaten with both eyes stabbed. He eventually lost eyesight in one eye and was left with only blurs in the other. From that moment he began to plot the death of Ippolito and Alfonso, and to succeed in this joined forces with another brother, Ferrante, who wanted to eliminate the duke and take his place. The conspiracy was clumsily conducted: the conspirators tried to kill Alfonso I on four different occasions with poisoned daggers. Twice they missed Alfonso, and twice they were too cowardly to dispose of their own borther. Ippolito soon discovered their plot: all the allies of the two principals were put to death, whereas for them, the sentence was reduced to life imprisonment in the dungeons of the Este Castle. In 1506 the two were shut up in the castle prison. Ferrante died in prison years later at age 63. Giulio was released released by Alfonso's grandson 53 years after he was jailed. During this time, neither men were ever vistited by any members of the family.