The Thieves Guild of New Orleans is a familial group of "extraction specialists" tied to the Benefactress, Candra. While Candra gave power to her Assassins Guild, she gave life to the Thieves Guild. A potion known as the Elixir of Life greatly prolongs the lifespan of the thieves -- Jean-Luc LeBeau is over 120 years old, and appears less than half that age.
A Unified Guild was created in Gambit (3rd series), but the truce & alliance apparently fell through, as the Assassins Guild has reappeared as a separate entity since then.