I know very little about "prelest" but from what it sounds like... a monk once recollected that a woman went to a meditation retreat and was doing well but apparently had been using the wrong method. Every time she meditated she would say, "wisdom, wisdom, wisdom" and it eventually drove her mad.
I wish I had more details on the wisdom that the monk ellaborated on but from what my teacher has relayed to me (I'm sorry I always use this as a reference but none of these are my original ideas):
Everything is generally an illusion of some sort... visualizing, sensations, even emotions you feel even in the context of buddhism. But this is not to be taken in the most literal sense. I'm not really an expert on this but it's more of a "everything comes and goes so what does it really matter?" that incites a sort of detachement from experience as well. It's useful to analyze but it's not useful to internalize to a great extent.
From what I understand, the only truth, and maybe spectrum opposite from delusion/delest/illusion would be the Dharma itself.... but from I understand.. enlightenment, nirvana.. samadhi.. are all "truth" and that is "knowing" ... and "no-self" and "non-existence."
Anyone have any other thoughts on this? I'm afraid I'm not doing the best job here