Arnold Schwarzenegger was originally to have reprised the role of Conan in this film, as the Red Sonja character was a spinoff of the original Conan novels. Ultimately, a new character was created who was essentially Conan in everything but name.
The original theatrical trailer of this movie contains cutting room-floor footage of scenes glimpsed only briefly in the film's prologue: Sonja, left to perish in her burning home, is saved by her dying brother Barlok; Gedren, prior to her disfigurement, addresses her soldiers and slaves.
The name of Ronald Lacey's character, "Ikol," spelled backwards is "Loki." Loki, in Nordic mythology, was the God of Evil and Corruption, infamous for being a jealousy-prone troublemaker, and plays a pivotal role in the End of the World.
It is commonly believed that Robert E. Howard created the character of Red Sonja in one of his Conan short stories. In reality, he created a character with a similar name (Red Sonya) that appeared in a minor role in a non-fantasy work of historical fiction. "Red Sonja", the character popularized by the 1985 movie, actually first appeared in a Conan comic book written by Roy Thomas and illustrated by Barry Windsor-Smith.
Arnold Schwarzenegger himself eschews this movie as one of his worst, yet he (laughingly) claims that it's an excellent disciplinary tool for his children: "I tell them, if they get on my bad side, they'll be forced to watch Red Sonja ten times in a row. Consequently, none of my kids has ever given me much trouble."
Arnold only signed up for nothing more than a glorified cameo, as a favor to producer Dino De Laurentiis. Much to his surprise, he was stuck on the set for four weeks, three weeks longer than expected, and discovered after watching a rough cut of the movie that his role was basically filled out to that of the co-star, thanks to crafty angles and multiple cameras being used. Schwarzenegger decided not long after, to try and terminate his 10 year contract with De Laurentiss.