So many vows. They make you swear and swear. Defend the King, obey the King, obey your father, protect the innocent, defend the weak. But what if your father despises the King? What if the King massacres the innocent? It's too much. No matter what you do, you're forsaking one vow or another.
~ Jaime Lannister mocking the vows taken by knights in the Seven Kingdoms.
Dark Knights (not to be confused with a certain Batman) are Stock Characters of sorts in fiction, especially fantasy.

As their name implies, they are evil (or sometimes misguided) knights, high-ranking military people from medieval times (or a fantasy equivalent).

Knights can also be found in the modern age, but it is a title rather than a military position, and a modern Knight shares their medieval counterparts' traditional title of "Sir".