I'm reading through the last bit of Alan Moore's run in Swamp Thing and I just read issue #56, which I hadn't visited again in quite some time, and I noticed that this can be the inspiration or part of the inspiration for the issue of Knightmares with a dream construct of John Constantine. In the issue of Swamp Thing, Swampy ends up stuck in a world with no intelligent life and he creates a fake world copying the life he had on Earth. The construct of his that confronts him about what he is doing is John Constantine. Just like in Knightmares, this false Constantine appears as the voice of reason convincing the protagonist that he is trapped in a lie