Well I have beat alot of things in this game and one thing i loved about Morrowind is that you could join the Imperial Legion. Can I do that in Oblivion and if I can't why not?
I will get around to it eventually, someone else is making one who will in theory be done far sooner than me, because he is doing less and started earlier.
You can't join the imperial legion but you can become a knight of Cheydinhal and Leyawiin (they are militia not imperial legion though), Cheydinhal involves only 1 quest which is part of another, leyawiin takes 2 quests to join and you can get a income off it if you want to hit the same place over and over again. And you can also become a Champion in the Order of the Imperial Dragon by following the main quest, it is about as empty as the order was in Morrowind (remember, you become a Knight of the Imperial Dragon in highest rank of the legion, but nothing special happens). And there is also the Order of the virtious blood or something along that name, which I think is just a group rather than anything official.
...you become a Knight of the Imperial Dragon in highest rank of the legion...
You become THE Knight of the Imperial Dragon, not a Knight of the Imperial Dragon. But, to answer the question, you can kind of become a guard, with... ah, man, it's not there anymore!
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