Posted by: crazyfoomandude
Actually, I don't think you need all of the original gamerscore. I have the Katana but I don't have Marathon Man achievement and the double laser achievement.Folks to clear up the confusion...
When Halo 3 was released, you needed all 49 achievements. The ones that shipped with the game- the only ones there were. In 2008 when the Legendary Mappack came out, it screwed everything up.
This situation continues today. Now, you can unlock it anywhere from 300GS-1750GS. There really is no set score. It is not 1000GS, that is a completely false rumor started on the basis that the original Halo 3 achievements, like every game, total 1000GS.
You will unlock it randomly. Maybe it will be at 1000GS, but that's only chance. You're just as likely to get it at 1300GS. However, with this glitch came another one. The armor permutation of the katana and Security helmet RElock themselves for a lot of people. Why does this happen? This happens because those people do not have the original 49 achievements.
So no, you don't necessarily need the original achievements to get it. However, if you don't, there's no guarantee they won't just disappear. Some people are lucky and never have them disappear regardless. Most aren't that lucky. So the safest bet is to just tell people to get the original 49 achievements, because it will secure them and make it so you don't have to type out a long explanation like this one every time someone asks.