He was the inventor of the Corvix Blade, a modified variant of the Hidden Blade which he was forced to design for the Templars by Cesare Borgia. His conscience compelled him to eventually defy Cesare, and after losing his tongue as punishment, he attempted multiple times to kill Cesare to no avail.
Around 1501, Demetrio was forced by Cesare Borgia to replicate the design of the Hidden Blade iconic to the Assassins for Templar use. For this, he was given the original blueprints of the Hidden Blade stolen from Leonardo da Vinci. After modifying the design based on a copy he made of the blueprints, he proceeded to craft a prototype using a stolen set of Hidden Blades, creating an upgraded version he dubbed, the "Covix Blades".
When Demetrio refused to continue creating more weapons for him, Cesare cut out his tongue. This, alongside the days of torture he suffered afterwards, caused the vengeful Demetrio to plot against Cesare. His murder attempts failed time and time again, though he escaped retribution initially.
On his final attempt in Florence around 1506, he sought the help of the Assassins, sending a letter to Niccolò Machiavelli one morning asking for an armed escort during his escape. He did not, however, enlist the Assassins for aid on the assassination itself, seeking to do it personally.
That night, he failed to kill Cesare yet again, and this time, Cesare discovered the plot and arranged for his execution. Fortunately for him, the Assassins Lo Sparviero and Niccolò Machiavelli had been watching over the Piazza della Signoria and saw him and other prisoners being dragged to the gallows by Cesare. Right before he could be executed, Demetrio was saved by Lo Sparviero, then taken into custody by Niccolò Machiavelli.
There, Demetrio confirmed that the modified Hidden Blades recovered by Lo Sparviero from a chest stolen by the Templar Pietro de Galencia were indeed the Corvix Blades he crafted.