No matter how old Fugu gets, he still lives in a world of illusion.
I sometimes cherish the sweet illusion that Koichi is my son.
My older brother maintains the ridiculous fantasy that he is the king of the blowfish.
An illusion that changes your concept of the world is a big fantasy (it's also an illusion). This makes sense, though, right? An illusion is an illusion, sure. But, when it changes your concept of the world, it becomes a total fantasy!
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.