The young bridegroom lay sprawled across his bed. Barely healed from the vicious stab wounds that had already brought him close to death, it was clear he would not recover from this attack. He had been strangled.

Alfonso, Prince of Naples, had fallen victim to one of the strangest and most obsessive relationships in history – the love of Cesare Borgia, the illegitimate son of Pope Alexander VI, for his beautiful sister Lucrezia.

If there had been gossip magazines in the 16th Century, the Borgias would have grabbed all the headlines. The Italian Renaissance was a time rich with poetry, art and invention, but it was also the time of the `secular’ popes. These fathers of the Catholic Church fathered illegitimate children, murdered their enemies, and broke most of the Ten Commandments, believing themselves to be spiritually immune from sin.

Of them all, Alexander VI, a poor Spanish Cardinal from Valencia who had risen to the most powerful office in the Church, was surely the most corrupt.

Alexander openly acknowledged his mistress, Vannozza, and the many children she bore him, as well as those born to other women – but it was the children of Vannozza on whom he lavished his love, especially his son Cesare and his daughter Lucrezia.

This love even surmounted the certainty that Cesare was responsible for the murder of his brother Giovanni. It was the first sign of Cesare’s obsession, the jealous desire to be first with those he loved – he could not bear even the rivalry of a brother for his father’s patronage.

That patronage was precious to the ambitious Cesare – as the son of the Pope, he had wealth, position and power. Without his father’s protection, he was just another boy born on the wrong side of the bed sheets.

But such was Cesare’s pride that he never gave this much thought. The despot son and his equally despotic father ruled over Rome with iron fists, ruthlessly sweeping aside anyone who got in their way. The Borgias took the philosophy of `family first’ to extremes – the family that kills together, chills together.

The murderous deeds of the Borgias are legendary. Their favourite method of disposing of bodies was to throw them in the River Tiber after a good stabbing, but they also made good use of white arsenic (known as cantarella) liberally sprinkled in their enemies’ food and wine.

Cesare di Borgia deserved his reputation as a ruthless despot. He raised an army with his father’s blessing and took possession of many smaller Italian states, intending to set himself up as King of Italy so his power would survive his father’s death.

But even while he was busy pillaging and conquering, his passion for his sister remained more passionate than brotherly. He had killed both her first lover, rumored to be the father of her child, and her second husband.  Her first husband, Giovanni Sforza, had been lucky to escape with his life through a convenient divorce. When Lucrezia was whisked away to Ferrara to become the wife of another Prince Alfonso, her third husband wisely gave her as little attention as possible, except to get her pregnant. Even hot blooded Cesare saw the necessity for this, and Alfonso the Second was spared her brother’s wrath.

Rumor abounded about the closeness of this family – Lucrezia was widely believed to be not only her brother’s lover, but her father’s as well. Scandal and gossip surrounded the pretty young woman, which her brother clearly did nothing to prevent. Like one of today’s `It’ girls, Lucrezia’s clothes and mannerisms were slavishly copied. Scholars have been divided on her complicity in her family’s plots, but today it is widely believed that she was a naïve tool, used by her father and brother to further their ambitions.

Nevertheless she adored them both. Even with the blood of her lover and husband on Cesare’s hands, she remained a loyal and devoted Borgia sister and daughter.

But history offers no greater irony than the downfall of Alexander and his son. They amassed much wealth poisoning wealthy cardinals, as Alexander was then able to claim their wealth for the Church. But it finally went hideously wrong when they drank the poison intended for one of their victims. Cesare fought his way back to health, but Alexander succumbed. His death left Cesare politically naked before he had managed to realize his plan to become King of Italy.

Cesare was quickly imprisoned by the new Pope. Rome had had enough of Borgias, and he found himself friendless and alone. But you can’t keep a bad Borgia down, and Cesare did manage to escape his imprisonment in Spain. He hoped again to raise an army and conquer Italy, but Cesare’s star had fallen, and he died in battle at the age of 31. What poison, hatred and plots had been unable to achieve, a sword thrust brought about.

Freed from the obsessive love of her father and brother, Lucrezia finally found the peaceful and pious life that had eluded her. But not for long – she died giving birth to Alfonso’s fifth child at the age of 39.

The cruel, murderous reign of the Borgias passed into history. But our fascination with this extraordinary family remains. Did the love Alexander and Cesare felt for Lucrezia go beyond decency? Only Lucrezia knew – and like her contemporary, the Mona Lisa, she remains enigmatic.