Collocations for delusion - noun

dangerous
paranoid
collective, mass, popular (esp. AmE)
He dismissed the so-called miracle as a collective delusion.

get, harbour/harbor, have, suffer, suffer from
The psychiatrist said she was suffering from paranoid delusions.

under a/the delusion
He seemed to be under the delusion that he would make his fortune within a few years.

delusion about
He had no delusions about his feelings for Kate.

delusion of
She had delusions of persecution.

delusions of grandeur (= a belief that you are more important than you actually are)
Don't go getting delusions of grandeur.