I'd recomend at lvl 13, because you get an extra 1/2 attack per round. If you add tob, i think 21 or 22 is what you want. You'll be restricted to thief weapons, and the only two handed kinds of those are staffs, and i'd stay away from those, unless you want the cheeseiness of getting mutliple backstabs without hiding

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If I asked, would you answer? Its your problem. Its a deep, deep problem. I have no way to ask about that... I have no elegant way of stepping into your heart without tracking in filth. So I will wait. Someday, when you want to tell me, tell me then. -Bleach