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The Herald was a gigantic, monstrous version of the Thirteenth Doctor from a potential future in which she was trapped in the Catastrophia and subsumed by its chaos. In the Catastrophia, the Herald existed alongside the Sanity and the original Thirteenth Doctor. (COMIC: Mistress of Chaos)
The Herald was imprisoned for three hundred years under a castle in Bohemia. (COMIC: Herald of Madness)
When the timeline of the Herald's creation was prevented by the Thirteenth Doctor, the Herald and the Sanity merged with the Doctor. (COMIC: Mistress of Chaos)