delusion

a fixed false belief or serious mistaken conviction

Roots

de-: down, away, reversal, reduction, or removal
lud / lus: play, mock, or trick

Other words with the lud / lus root

allusion: an indirect reference
allusive: hinting indirectly at another idea or text
illusion: a false appearance or mistaken perception
illusory: seeming real but false or unreliable

Delusion is a false belief that plays the mind false.

The easy first answer was a delusion; the sentence actually pointed the other way.

De- can carry the sense of detriment; lud gives play. Delusion is not playful in tone. It is the mind being played false until a false thing feels settled.