An organisation that houses Assassins. The guild supplies the assassins with a place to live, meet colleagues and store supplies.
It is also the medium through which an assassin is contacted and contracted. Assassins Guilds often make a point of being politically neutral which means that assassins from the same guild wind up serving opposing clients. This comes with the territory and is not seen as bad in the least.
The symbol of the assassins guilds across the entire world is the scorpion.
For the drow-assassins this is seen as a sign of Lolth's favour since the humble, deadly scorpion is an arachnid.