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The Bandrils' relationship with the Karfelons was sometimes friendly though occasionally there were moments of tension between the two neighboring races. The Bandrils subsisted in part on grain from Karfel. (TV: Timelash)
In 2018, (PROSE: Lucy Wilson and the Bledoe Cadets) the Bandrils attempted to influence the minds of children so that they would kill their parents, aiding in their takeover of the Earth. To do this they created The Billy Bandril Show, which they used to influence human minds into buying the Billy Bandril toy. Lucy Wilson and Hobo Kostinen uncovered this plan, and altered the Bandril's broadcast so that the children of Earth destroyed their Billy Bandril toys, ending the invasion. (PROSE: The Bandril Invasion)
Lance Parkin has stated that the Bandril invasion reference in The Dying Days was a joke, based on the Bandrils being "rubbish" monsters, and that "some alien invasions were beneath the Doctor's dignity to deal with".