And automation.. If the light effect moves around, it's nice to change its sound with an automated filter.
As for the imaginary part: what is it for, what does the story tell, is it good or evil, real, or phantasy?
You could try (a combination of) water, wind, welding tools, snake hisses, breaths, tones like from a triangle.. and some envelope shaping of course..
What does it sound like in your head? Now, working backwards, try to determine how you could go about achieving that sound. How would you characterize its various sonic elements, e.g. Its envelope, timbre, pitch, etc. What real life sounds does it resemble?
Or you could take the opposite approach and just grab a bunch of elements that sound interesting and manipulate/layer the hell out of them until you get something cool, even if it's not remotely what you imagined.