Impossible for consciousness to be an illusion, since if it were then who or what would be subject to that illusion. But it is quite possible that the nature of consciousness is illusory, which means only that there is nothing that it is like. It is not like light, or vision, or intelligence or any functioning of mind. And for the same reason it is not possible to say that it exists, since if it did it would be delineated in some way, would have a boundary, and this is not possible either since for whom or what would that boundary be drawn. The only conclusion is that consciousness is everything, not in the sense that it is the stuff out of which everything is made, being the nature of appearance itself, but like the universe of sets which cannot be a set. When you think about numbers there is a sort of symmetry between zero and infinity, technically via a compactification. No such symmetry operates between the empty set and the universe of all sets, and so between the nothing which is integral to the existence of any thing and the universal which is consciousness.