It was very situational, but it'll forever live on as one of my fonder gaming memories.
The setup: The PCs are infiltrating the hidden temple compound of this large, particularly fanatical death-god cult, here to save the professional sacrific...er.. innocent plot-pertinent princess who everyone and their mom keeps trying to snatch away for their own nefarious ends. All the cultists are in the main chapel, praying up a storm while their high priest whips them into a frenzy with some wicked-awesome sermoning. The rogue/sorcerer is sneaking around to the far side of the altar to the door leading to where Her Royal Importance is trussed up and skimply attired. The wizard is out of line of sight behind some cover providing overwatch for if and when it hits the fan. The rog/sor is just passing the dais when it happens.
The delivery: The preacher is preaching his black heart out. The cultists are eatting it up, praying like the world is about to end. Hell, praying THAT the world is about to end.
HIGH PRIEST - ...and WE the Black Lord's chosen people shall flow forth like a righteous tide into the streets and homes of *etc, etc, etc*
CULTISTS - *praying, moaning, rocking back and forth, etc*
WIZARD - *lightbulb appears over his head!*
WIZARD - *from his concealed location, casts Major Image, his casting going unheared over the already burbling torrent of voices in the chapel*
MAJOR IMAGE - *image of a giant, charred corpse's chest and head pushing thru the shadows at the ceiling of the chapel like a caul, the likeness in-line with any of the roughly 8-billion icons of this death god strewn about the place*
MAJOR IMAGE - *deep yet rasping voice* 'Lo, my children... your voice has reached beyond the black sands of nothing to grace my thoughts, and I come before you to answer those prayers. Heed my call, the call of the destiny of all things. Join with me in final baptism and I shall make you the instruments of my will, death undying, to cleanse all the world.
MAJOR IMAGE - *giant dagger (deity's favored weapon) descends from the shadowy ceiling, shrinking as it goes, to point directly at the high priest's heart. Every cultist is carrying a dagger just like this one on them*
HIGH PRIEST & CULTISTS - *dead silence, enraptured with religeous extacy*
ROGUE/SORCERER - *hidden near the altar and High Priest, still unnoticed by everyone*
ROGUE/SORCERER - *Casts a silent Suggestion on the High Priest, who, thankfully fails his save* Follow your god.
HIGH PRIEST & CULTISTS - *all commit suicide*
...it was beautiful.