Saiko Egawa

After a slumber of twelve millennia, the Demon Empire awakens to seize control of the Earth. Raideen, the giant robot-like protector of the lost continent of Mu, senses the evil presence and awakens within its golden pyramid. A young Japanese boy, Akira Hibiki, is alerted about the Demon Empire by a mysterious voice and rushes to the pyramid. It is soon revealed that Akira is a descendant of the ancient people of Mu who must help Raideen save the Earth. (wiki)

Kouji and his friends have defeated Dr. Hell and are now enjoying a break, but suddenly a strange prophet appears and warns everyone of an oncoming danger, mechanical beasts never seen before start appearing all around the world wrecking havoc. Its up to Kouji and his Mazinger Z to stand up to this threat but it seems he is vastly outnumber and outmatched. This film served as an alternative link between the Mazinger Z TV series and the Great Mazinger TV series. (myanimelist.net)