Not necessarily, Cooljoe. I've seen plenty of games that have DLC achievements which refer back to the original game. It gives it some replayability!
To be honest, this achievement isn't that hard. Just time consuming.
1. Assassin's Creed (1000/1000)
2. Assassin's Creed II (1000/1000)
3. Assassin's Creed: Brotherhood (1250/1250)
Working On - AC:R, Skyrim, and Portal 2
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