After defeating the giants, the Guardian Brotherhood stood alone in the Hall of Lolth. There was a desecrated altar to the Spider-Queen, clearly their job was to consecrate it.
They all took turns doing what they could, but eventually, it turned out they were just not prepared to fix something this intricate, and the altar was ruined forever. This, in turn, called the guardian of the altar – a Drider!
Along with it’s minions, the Drider attacked the heroes, and after they finally defeated her, they found a token of Lolth left behind. This must be what they were seeking.
They took the purple gem back to the Priestess in Quor’Toth, and she told them the meeting place in Westfall where they could find their contact, who would then enact the ritual to poison the water system. They were given a sealed box to deliver to the contact, as well. They had other plans.
They traveled back to Sigil, and went to their alchemist contact Niven. They asked to use his lab to try and open the box to see what was inside. Boswell, alone in the room, was unable to open it and was attacked by a giant spider.
After defeating the arachnid, the Guardian Brotherhood took the box to a magic shop to get a replica made, that they would take to the contact to fool them into opening instead. They prepared for their next meeting with the Drow.