You don't find the assassin's guild, they find you.
Where these dark outlanders came from, no one knows. One day, they arrived in Harbat on a ship with a black sail and took residence at a local nobleman's manor (speculations as to their allegiance have been abundant). They stayed with him for a while, after which they just vanished from the face of the rock. Their ship too hasn't been seen since. Where most people believe they have settled here to do business, insiders believe they are here to find a certain person belonging to a secret network. Something to do with revenge, repaying debts and blood oaths. These rumours remain unsubstantiated.
The assassins are a silent bunch, draped in silence and mystery. They maintain contact with one another through a complex system of coded notes left throughout the city. If ever you see a note on a public board that reads "man searches cat to feed", chanches are this is a secret message. Then again, it might really be a guy in a dire need for a pet.
They operate in silence and mostly, so are their tactics. Poison is their most common weapon. However, if the target has tasters, or if the client wishes the victim to know someone has ordered his life taken, they will use physical precision by means of gags and daggers. This costs a little extra though, because of the increased risks.
The assassin are thorough professionals who know their business in and out. They are excellent fighters with a very specific, graceful fighting style. They circle their opponent at a running pace, will suddenly make a dash towards him, leap into the air and either hit him hard on the shield or armor to throw his balance or make a faint and cut him in his side, his arm, his leg. In duels they never go directly for the kill. Instead they play with their foes. Draining the life from them, cut by cut.
However, this is not the main reason why people fear them. Everyone knows that to upset the assassins, is to live in fear of death for the rest of your life. No matter how long, they will find you. Whether by blade or poison, death will be on you sooner or later. Never again will you be able to enjoy a meal or a late-night stroll. Never again will your sleep be free of angst and demons. To strike against the assassins is to abandon all joy for life.