They confirmed that the summoning itself had succeeded through the [spirit board]. But they couldn’t confirm if the Master was Sagara Hyouma.

In other words, there was a high probability that someone besides Sagara had become the Master. Assassin of Black—Jack the Ripper—had a short history and was a serial killer, which was far from the level of a Heroic Spirit, but with the special characteristics of his class, he undoubtedly specialized in killing Masters.

- Vol 2 Chapter 1