I like the story more each time I read it.
There is genuine dramatic tension- Ortwin dead, Iua down, Mostin gone gods-know-where, Eadric feebleminded, Nwm hallucinating in a corner, Tahl being energy-drained... it looked as if a TPK was at hand.
Thank goodness for Mulissu!
I wonder- could the simulacrum of Graz'zt have gated the real Graz'zt in? *That* would have been scary!
Ridley's Cohort: IDHTBIFOM but I think succubi can cast
clairvoyance every round. They couldn't all possibly be dispelled. But you are right that Mulissu would be undetectable.
And I think that teleporting away until the buff spells wear off could work- someone on the demon's side could maybe cast a
cloudkill in the ventilation system so that the invaders wouldn't be able to snoop around easily-
darkness spells and glyphs/magical traps would help too.