The Thieves Guild is an international organisation for thieves with headquarters all over the planet, though the only current guild hall known is on Terror Mountain (there is a disused one near the Haunted Woods). Little is known about the Guild, but it is currently headed by Kanrik, who took the position by force from Galem Darkhand. Access to guild halls is provided by a pendant, shaped like an emerald being encircled by a golden Cobrall. The guild is known to recruit in Portal City.
In addition to the named members of the guild, numerous background characters appeared in the Hannah and the Ice Caves plot and TCG set. Perhaps most notable are a Korbat and a Techo, who had lines in the plot:
The Desert Scarabs are not part of the Guild, but have previously been used as a recruiting ground for them. The status of Purrow and Meesha within the Guild is under question.