Well, there was the time that a friend was doing his DM thing and was planning on being completely serious - Everything we said was "in-game". That was a mistake.
We were up against a demon called "Neighbor Finnian" (Funny enough by itself), and we gave him a trapped scroll (He asked for it - literally!). It zapped him with electricity that left his hair standing straight up.
Friend says, "You look just like Don King!"
DM says - In character - "And who is that?"
Friend instantly replies, "The god of bad hair..."
Needless to say, the session ended there as all seriousness was over...
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Of course, there is also my time as DM and the party encountered the "Divine Yum-Yum". It was a mangy malformed stray cat that some cultists were using as a "sign" from their mad god. In reality they just made up whatever they wanted for its "divine words" so that they could control the lesser cultists.
Of course, the PCs decided to take advantage. They used magical ventriloquism to make it talk, then to keep the handlers from changing things, they pumped energy into the critter. It started bouncing around like a hyperkinetic "Super Fun Ball" (TM)... The exploded in a burst of flame and fur. My poorly chosen words that ended the session for that night?
"Poof goes the Divine Yum-Yum".
Not quite sure why they found it THAT hilarious, but it cracks them up to this day...
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Smoss
Doulairen (Doulairen)
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RPG System (Doulairen)