Bnapsen is one of the seven Heptarchical Kings that John Dee recorded in De Heptarchia Mystica, the treatise drawn from his angelic sessions with the scryer Edward Kelley across 1582 and 1583 and preserved today as Sloane MS 3191. Bnapsen rules Saturday and, by the system's own day and planet correspondence, stands under Saturn. His attendant is the prince Brorges, and together they form the seventh of the Heptarchia's seven king and prince pairs, one assigned to each day of the week.
The Heptarchia is a separate stratum of Dee's angelic material from what most modern readers mean by Enochian magic. It predates the forty eight Calls that Dee and Kelley received later and that Meric Casaubon printed in 1659, and it predates the ninety one Governors of the thirty Aethyrs that occupy most twentieth century Enochian practice. Where the Calls and the Aethyr system organize the angelic hierarchy by elemental Watchtower and by aethyric grade, the Heptarchia organizes it by the seven day week and its planetary rulers, tying Dee's early angel work to an older current of Renaissance magic in which each day carried its own ruling spirits.
Unlike the Calls, the Heptarchia was never printed in Dee's lifetime or in Casaubon's seventeenth century edition of the diaries. It survived only in manuscript until Joseph H. Peterson's modern transcription and edition made Sloane MS 3191 broadly available, which is part of why Bnapsen and the other six Heptarchical Kings remain far less familiar today than the angels invoked through the later Calls.
Sources De Heptarchia Mystica (Sloane MS 3191), Joseph H. Peterson's editionJohn Dee, edited by Joseph H. Peterson with Arguing with Angels: Enochian Magic and Modern OccultureEgil Asprem