Be me, newby DM running a oneshot campaign for some friends.

Be not me, Tiefling bard, Gnome Cleric, Halfling Rogue, Dragonborn Paladin, Aarakocra Druid and Dragonborn Barbarian (all level 5)

Party is in weird creepy dragon temple thing

Party finds huge cavern

Bard heard weird slithering noise, looks up to see Young Green Dragon(buffed) staring at his face.

Entire party stays so calm it's unbelievable

Dragon drops to the ground and says hi

This shifty looking mofo has a necklace of weapons and a huge scar across his eye

Dragon makes up some bullshit story about how he doesn't kill people anymore, just collects weapons

If they give him their most used weapon each he'll let them pass

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Swears an oath on a god he doesn't even follow

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MFW entire party falls for it completely

Not one person thinks the dragon colour known for being sneaky and is currently asking them to disarm themselves is a little off

MFW entire party hands over their weapons

MFW entire party proceeds to group up on a "pressure plate" to open the door

MFW four people fail their saving throws for the Dragon's Poison Breath and are instantly KO'd

MFW I have to screw up my entire plot and reveal another NPC was a good dragon all along to save their asses from the Dragon

MFW entire plot falls apart

MFW entire campaign ruined by my player's IRL intelligence scores of 6