A malevolent presence was detected in the Seattle Undercity, a place both beside and underneath Seattle Metro.
After lots of meandering about and getting turned around they found themselves in an 'alternate' Columbia Tower. They then walked to the top floor, where they found the foolish Diabolist who had been taken over by a fell presence, but not before he has warded himself.
The group broke the warding, causing an titanic melee which they won - killing the foolish magical dabbler in the process.
Oh yeah, and Kerrigan was dead for a bit, but Simon figured out a way to supercharge her corpse to keep her wandering about - eventually causing a side trip to get her 'repaired'.
The group finally escaped the Undercity thanks to the help of a lost Homeland Security analyst who had wandered into the parallel space by spending to long on the wrong toilet.