Gabriel Rosewood

 was the leader of a cult. In 1965, he had encountered the "Highgate Vampire", which was actually a creature called a Corvid. He craved the taste of power he held when he was held in the creature's grip, and by 1972, seven years later, plotted to use his followers as bait to gain the Corvids' power. In 1972, the Corvids attacked and drained the psychic essence from several of Rosewood's followers as they gathered in Highgate Cemetery, and subsequently turned on Rosewood himself. (COMIC: The Highgate Horror)

Six months later, Jess Collins told off the Twelfth Doctor for running off in the TARDIS after Rosewood tried to kill them and having suddenly come back. The Doctor recalled preventing Rosewood from unleashing the Corvids and nearly killing the Doctor and Jess, but not Clara Oswald being present, (COMIC: The Pestilent Heart) due to the Doctor's memory of Clara being wiped by a neural block. (TV: Hell Bent)