Harry A. Earnshaw

Megalomaniac and would-be world dominator Roxor has kidnapped Robert Regent, along with his death ray invention, in hopes of using it to degenerate humanity into mindless brutes, leaving himself as Earth's supreme intelligence. Faced with revealing the machine's secrets or allowing his family to die a horrible death at the hands of Roxor... (imdb)

Chandu consults his crystal ball and sees that Nadji, Princess of Egypt, is in danger. She is about to be sacrificed by the black magic cult of Ubasti. Headed for the magic island of Lemuria, he is shipwrecked , washed ashore and captured. He becomes invisible, escapes and after numerous detours is able to rescue the princess. (imdb)

On the mystic island of Lemuria, the cult of Ubasti seek the Egyptian Princess Nadji to sacrifice so that their goddess Ossana, whose soul resides in Nadji's body, may be resurrected by Black Magic. (imdb)