The Arcanis Servitorus was Thomas Vaughan's personal army, created to keep the secret of his immortality safe.
In the 21st century, they posed as members of Azureus Security, a security firm headed by Vaughan's current identity, Sir Robert Craig. He had them kill Sir Giles Walbridge and Alice Proctor, Megan Brook and the other employees of the CARU Clinic, and Ella Contas.
They also almost succeeded in killing Ianto Jones, Gwen Cooper and Owen Harper. After Craig's death, the other members of the Arcanis Servitorus guarded his young clone, Simon Craig. (AUDIO: Hidden)