It is actually unclear whether or not Krystal actually exists, even in its own context. It may be that it is simply a mixture of whatever drugs Wallace Sage had to hand when he tried to commit suicide – a cocktail of aspirin, acid and whatever else he could find. It may be that it is a substance powerful enough to project the user into another context entirely. It may be that is nothing more than a syringe full of green ink.
Whatever it is, taking it is invariably fatal, unless it isn’t, and tends to make one either hallucinate that one is a superhero and that other superheroes are also real; or, more disturbingly, actually project you into a world where you are a superhero alongside other superheroes. I’d explain it more clearly, but that’s the magic of Grant Morrison.