How does The Night Circus relate to Frankenstein, thematically?
There is a sense in which this novel can be related to Frankenstein through the way in which Victor Frankenstein births a creature in an act of scientific discovery that has far reaching ramifications and leads onto unintended consequences that he was never able to imagine or predict. For Victor, there is a definite sense in which the creature he creates comes to haunt and blight his existence, killing those nearest and dearest to him and even deliberately seeking to make his life a living misery. In the same way, the conflict between Prospero and the grey-suited gentleman, which has been raging throughout the centuries, comes to dominate itself in ways that were never anticipated or expected. The grey-suited gentleman himself admits as much at the end of the novel when he speaks to Widget:
You have no idea what you are getting into, my boy... How fragile an enterprise it all is. How uncertain the consequences are.
Both novels are therefore linked thematically through the creation of something--whether it be a creature or a conflict--that surprises the initial creators because of the consequences of their act of creation. It is impossible to predict how things will develop and map out, and it is clear that this is a key theme in both of these texts. The Night Circus is something that involves so many people and has had far-reaching consequences, and in the above quote the grey-suited gentleman reflects on the way that it has become something much bigger and much more expansive than he had ever imagined. In the same way the creature Frankenstein gave life to comes to have massive implications and unforeseen consequences on the life of his creator, and the apparently innocent act of creation becomes fraught with difficulties and ethical uncertainties. This is true for both texts.