sorry forgot to answer the duke.
talk to sir dante en say you know he has a link to the assassinsguild.
He'll arrange a meeting (after a small quest).
talk to the guy and follow him.
new arragment with killer and kaptain at barracs (if you want).
end
i didnt tell everything so i dont spoil it. But these are the major lines to follow. If you have mor questions, dont hesitate to ask.