While the Guild was working in Kelsler, the Illithids had a plan for the Guild. They needed to keep eyes on the Guild and on Starbridge but somethings was blocking them from using divination.
A team was sent into the city itself, disguised as refugees. They spent several weeks in the city, learning the layout and investigating the location. They discovered that large crystals had been brought into the city and stored in locations around the city. They had only found one location though and inflitrated the building. There they found the crystal set in formation and bound with Abjuration and Divination magics. They tore down the formation and took one of the crystals back to their Illithid masters.
As they left, they launched a pearl into the air. They watched it burst into a flare, lighting up the night. This flare signaled the attack for the forces laying in wait outside the city. The army of the Underdark swarmed the city, burning and destroying everything in their wake. The city was not utterly destoryed but that was not the point. The message had been sent. The real conflict was soon to begin.