Flaming Lantern (3.5e Equipment)
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A simple trap, but one that can be versatile with preparation. The container below the flame of a lantern has had easily overlooked holes put in just above the oil level. If the lantern is tipped or dropped the oil will spill out and set the whole thing ablaze. An adventurer who uses this lantern could find it bursting into flames in his hand, and might spill more oil over himself. Movement like running, climbing, combat, or leaving it swinging from a backpack will surely set it off.
You could also set it up with a simple mechanism to knock it off its wall hanging or drop it from the ceiling, causing it to smash open and splashing burning oil on all adjacent. In the right circumstances it could just be set down somewhere likely to be disturbed. Try positioning it over an oiled floor, in a room full of hay, or near grain sacks full of grain dust. This trap must be set just before the 'visitors' arrive, and for every second it burns it loses potency.