Basilides held an important position in Lord Asriel's ranks as the alethiometrist. After the angel Baruch revealed that Lyra Silvertongue was taken by Marisa Coulter to a cave in the Himalaya, he was immediately set the task of pinpointing the exact location as the Valley of the Rainbows.
He was later able to reveal to Mrs Coulter, who had been recovered by Asriel's forces, that Lyra had survived the bomb threat and that she and Will Parry had left the world of the dead. In a meeting with Lord Asriel, King Ogunwe, Xaphania and Madame Oxentiel, he explained how, from the alethiometer, he had learnt that the children were separated from their dæmons in the process. Metatron intended to capture them to control their future, making the recovery of the dæmons Asriel's top priority.