If you want every achievement then yes.
multiplayer achievements and skulls
Get sculls or get achievements
on an xbox press multiplayer
yes
Getting the katana sword would mean you would have to unlock every achievement in the game. So you would need both the regular Halo 3 disk for the Campaign achievements and the mythic disk for the online and multiplayer achievements.
Halo 2
No
yes everything except multiplayer
There is multiplayer in halo 4
Yes that's the purpose of the multiplayer disk. the odst disk has online multiplayer to.(4 player co-op campaine and fire fight)
Becuase it has amazing graphics and it is multiplayer and of course its online
Yes, Halo Wars is a Xbox Live game, therefore you can play online against other players.