Song of Aten's Arsenal

The Song of Aten's Arsenal was a Gendar legend. It spoke of the Sun Builder weapon Giles knew as the Greater Key, although it described the artefact as a silver rod, and gave it a different name than "the Greater Key". It related the possible dire consequences of using the Greater Key, claiming that it would act like a malicious genie, creating a timeline where activating the Key was not necessary, but not necessarily in the way that the Key's user would expect or want. Maxie summarised the Song of Aten's Arsenal to Giles when he showed her that the Greater Key had found its way to P.R.O.B.E.'s custody. (PROSE: Out of the Box)

Behind the scenes
Aten was a solar deity of Ancient Egypt. Thus, the title can be understood as a poetic way of referring to the arsenal of the Sun Builders, of which the Greater Key was an example. Alternatively, given that the Greater Key is implied to be the same artefact as the Moment, which was shown in The Day of the Doctor to be kept in the Omega Arsenal, "Aten's Arsenal" may be an alternative name for the Omega Arsenal, using "Aten" as another name for Omega.