black knight 
noun Stock Market
A person or company making an unwelcome takeover bid for another company.The black knight, of course, isn't the type to court a maiden with flowers but instead prefers a fast horse and a sack....
- Comcast has opened the door for possible black knights to appear too, in the form of Viacom, Liberty Media or even Microsoft.
- A white knight is a person who is asked to acquire a company as an alternative to an unwanted black knight, who is the investor initiating a hostile takeover bid.
Origin
By association with white knight.