The Psilent Songbox of Karn was an augmented Gallifreyan children's toy. (COMIC: Kill God) The War Doctor, the Master, and the Squire stole the Songbox during the Time War to help them defeat the Cyclors.
Alice Obiefune stole the Songbox from the Doctor and used it herself, banishing the Cyclors to a higher dimension. After the Master attempted to escape by flying a TARDIS from his future, the paradox that was created reacted with the Songbox, collapsing timelines together, resulting in complete chronal meltdown. (COMIC: Kill God) The Songbox reacted with the Chronal tumour inside The Master's TARDIS, which made it into a temporal bomb. It began to warp everything it touched, including the Cyclors.
The War Doctor used the Songbox to reverse the origins of a chronal-tracker, thus creating The Then and the Now. After he had successfully sent Alice home using the Then and the Now, he smashed the Songbox, to save everyone. (COMIC: Fast Asleep)