The phrase "harsh mistress" refers to a situation or person that may be demanding, strict, or difficult to please. It implies that dealing with or being under the influence of that situation/person can be tough and unforgiving. Full definition
Continuing with the roots of the company they announced two light novels, based on the wildly popular Attack on Titan property; Attack on Titan Before the Fall: Kyklo Arc and Attack on Titan: Harsh Mistress of the City. (manga.jadedragononline.com)
Attack on Titan: Harsh Mistress of the City takes place just after the beginning of the Attack on Titan manga. (manga.jadedragononline.com)
Science - fiction author Robert Heinlein didn't invent the phrase, «There ain't no such thing as a free lunch,» but his novel The Moon is a Harsh Mistress helped to popularize it. (fortune.com)