The Real Reason Why Assassin's Creed Black Flag Is No Longer Available On Steam - GameSpot

The answer is not very exciting or dramatic.
Assassin's Creed IV: Black Flag is a free-roaming action adventure game for both current gen and next gen consoles. Players will take the role of young captain Edward Kenway whose exploits earn the respect of pirate legends like Blackbeard, but draw him into an ancient war that may destroy everything the pirates have built.

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Assassin's Creed II, Assassin's Creed Wiki

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Update from a dev on Adding achievements, basic feature, to AC