The Vayne Portrait
This trap takes the form of an oil painting housed in an ornate gilt frame. The painting is a portrait, but the subject of the painting appears differently to each viewer. When initially encountered, the portrait is veiled by a sumptuous red curtain. The ground immediately at the foot of the portrait acts as a pressure plate; anyone stepping into that area activates a hidden mechanism that unveils the portrait.
Once unveiled, the portrait appears to be an aesthetically-pleasing image of the viewer. At first, they see themselves in the prime of their youth; even if they never possessed physical beauty, the painting shows hidden aspects of their comeliness that they never imagined possible.
However, the beautiful portrait soon turns foul and degenerate. Before the viewers' eyes they see the weight of their secret sins corrupt their visage. Each round that a person is exposed to the Vayne Portrait they must make a saving throw vs. poison or lose one point from an attribute. The first round a save is failed means that the viewer loses a point of constitution, on the second they lose a point of wisdom, on the third they lose a point of strength, and then the cycle begins anew. Characters who fail their initial saving throw are enthralled by the painting and unable to act as it siphons-off their mortal vitality.
If any of the viewers' attributes drop to 0 because of the portrait's effects they lose their humanity entirely and become a monster under the control of the GM. A character who passes their saving throw at any point may act freely and can break the portrait's power over other viewers by veiling it, obscuring it from view, or blinding the viewers. Note that attacking the portrait is ill-advised; any attack against the canvas means that the remaining enthralled viewers must make a saving throw vs. poison or die immediately—anyone who fails this save will have their body contorted into the wrecked visage they saw displayed on the canvas as a record of their hidden sins.