The Secret of the Sword!
From 1985, The Secret of the Sword was He-Man’s first and only animated movie. Though it was briefly in theaters, the movie was more widely seen on regular television. (As a kid, I don’t know if I was even aware that SoTS had a theatrical release!)
The movie introduced She-Ra and Hordak, and is probably most famous for the bitchin’ song that plays over the intro. Enjoy!