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You need the base game. If you don't know anything about the game, I would start with that first, and expand if you enjoy it.
At the risk of being a total fan boy, the base game is excellent. I tell folks who are curious, it's like a simpler version of risk (fewer spaces), but you have to bid on the ability to recruit and move fleets, recruit and move troops, get better bidding position and hunt for creatures, or just move forward towards some general VP. Only one player gets one of those actions a round. You might be in a perfect winning position, if you only could move some fleets, so the other players outbid you and play continues. So, if you like dudes on a map, and bidding (which I do), then look no further.