While tracking his targets through Europe, it so happened that there was a great battle between the infernalists he had watched and a group of what he believed were Camarilla members. After the fight, several lay in torpor, and Isaac, being the ever curious individual, took one to study.
He had chosen what he later would find out was a Toreador Elder and he dissected the individual studying how the kindred body worked first hand. Once he'd learned as much as he could about the individual, and being one that didn't believe in wasting anything, he consumed its soul. This of course he would later note in his journal would be a quite invigorating practice that he would like to engage in later for... Scientific purposes.