The thieves guild of Aerta, run by the mysterious Queen of Birds, may very well be older than the Imperium itself. It's exact membership is unknown. Arrador and Delg both served as members, though they both have since left their ranks.
The rules of membership: Don't steal from another Kestrel. Don't act in violence against another Kestrel. Arrador intends to break the second rule, for Delg's breaking of the first.
Every town has a kestrel contact, and safe house, if you know how to find them.