Black Flag has it's share of language and sexually suggestive scenes. The core gameplay is still killing in a multitude of gruesome ways.
Assassin's Creed IV: Black Flag is set before the events of Assassin's Creed III. You play as Connor's grandfather, Edward Kenway. The story starts in England where Edward is wanting to find fame and fortune. He leaves his wife for a life of privateering. Edward ends up spending most of the game playing the Templars against the Assassins. He only chooses the side of Assassin's very late in the game. I don't want to ruin the whole story for anyone who hasn't played the game.