Here is what I'd like to propose - thoughts welcome:
I shall become indispensable to my guild, to the League, and to the Empire. In the marketplace, on the battlefield and everywhere, we are bound by blood and brine.
I'd particularly like something bettter than "everywhere", which just feels a bit weak, but I would like the oath to be inclusive without getting unweildy.
I shall become indispensable to my guild, to the League, and to the Empire. In marketplace and workshop, at peace and war, on land and water near and far, we are bound by blood and brine.
Poetic, but we need to keep it short and snappy, so I can remember it when new people want to take it (as they naturally will when they discover how utterly awesome we are). So, how about (for the second sentence)
"At home or abroad, in trade and at war, we are bound by blood and brine."