"Cruel mistress" refers to a situation or person that is demanding, unkind, or takes control over someone's life in a way that can be difficult or painful. Full definition
If only Joan Bennett & Alida Valli, the wickedly cruel mistresses of the dance academy, were still around for cameos... (thefilmexperience.net)
Oh LG.Philips, you are a cruel cruel mistress. (gizmodo.com)
It seems that irony is a very cruel mistress as just last week we wrote about how NCSoft with specific regards to Aion and how they're advocating security across the board. (thatvideogameblog.com)