Megalon is a gigantic insectoid being that lives deep underground in the subterranean kingdom of Seatopia. Megalon is seen as the Seatopians' god, and is called upon by them to defend their home from threats.
Thanks, creature feature posters are the best thing ever.
And yes, all these movies are real. The japanese posters are a bit over the top and the posters are far more epics than the films, many of these are boring low budget syfy things.