For a total of $19.99 you will be getting access to all the DLC that will be released for the game at a discounted price, thereby saving 20 dollars. You’ll also be getting access to extra single-player missions and multiplayer content.
The description on the AllGamesBeta post also reveals some interesting details about the DLC. Apparently you’ll be able to play as Adewale, the main protagonist of the game’s first mate.
“..the adventures of his first mate, Adewale, have never been revealed. Until now. Pre-order Assassin’s Creed IV: Black Flag Season Pass now for the chance to play as Adewale, a new assassin who will take you even deeper into the real explosive events of the Golden Age of Pirates.”
Assassin’s Creed 4: Black Flag focuses on the adventures of Captain Edward Kenway, a pirate-turned-assassin. You may recognize the surname Kenway, as Edward is actually the father of Haytham Kenway, and the grandfather of Connor from Assassin’s Creed 3.
Assassin’s Creed 4 is due on PlayStation 3, Wii U and Xbox 360 in late October and early November, with the PC version to follow in a few weeks. The PlayStation 4 and Xbox One releases depend on the launch of the consoles themselves.