An illusion of that size would not be effective on the population at large. The illusion would need to be more tailored to a person's individual life and experiences to really cut through.

The one exception would be recreating an illusion of one of the Confluences that occurred, as the citizens of the city would have experienced that and will have an emotional reaction to it.

As for blowback, we will leave that up to you! For guidance, we can suggest that creating an illusion of the confluences and maintaining it long enough for it to have an effect may put him a medically significant amount of strain.