It'd have to be done through stats somewhere, I assume.

How about a set percentage of their health?

Like, in this case dealing about 1/5 of their HP...

IF
SomethingThatRecognizesTheTrapIsSprung
AND
DB_IsPlayer(_Player)
AND
CharacterGetHitpointsPercentage(_Player, _HPPercentage)
AND
IntegerDivide(_HPPercentage, 5, _NewPercentage)
THEN
ApplyDamage(_Player, _NewPercentage, "Chaos");