Zarn was infected by with the vampire genes while visiting the Three Who Rule in their castle. He returned to his lands where he infected his fellow villagers. There were soon several vampires in his dwelling.
He made regular visits to the castle and found favour with the Three. After they were killed, Zarn sought revenge on their killer, the Fourth Doctor. He had Hurda seek him out, but instead she brought Romana II, the Doctor's companion.
The Doctor soon followed and was taken prisoner. Romana escaped and the vampires nearly killed the Doctor against Zarn's orders. Romana returned with the Eighth Doctor and they made to rescue the Fourth. However, Zarn found them. As he moved to attack, he was impaled from behind on the pike of Ivo. (PROSE: The Eight Doctors)
The Eighth Doctor later recounted to a confused Adrienne Kramer (who hadn't the faintest idea who Zarn was) that he regretted the killing Zarn and his followers, deeming killing vampires "the easy solution" and stating that he had only abetted it in his fourth incarnation because he had been, at the time, too young to know that "that's not what I do". (PROSE: Vampire Science)