As of June 2019, no plot has been announced. Sonja's backstory in the Marvel Comics is a grim one as it established her as a 17-year-old woman living in the fictional Western Hyrkanian steppes with her father, mother, and two brothers. Sonja is turned into "the She-Devil with the Sword" after a group of mercenaries kill her family, physically assault her, and take all of the family's possessions. A broken young woman, the red goddess Scathach endowed Sonja with the incredible skill to handle any weapons, especially swords, under the special provision she would never be with a man unless he defeated her in fair combat.
Based on the fierce 6-foot tall female warrior character Red Sonja, who first appeared in Marvel's Conan the Barbarian comics back in 1972, Red Sonja soon became a popular enough character to warrant getting her own Marvel run. Dynamite Entertainment helped revive interest in the Red Sonja franchise in 2005 and continues to publish new stories about the She-Devil with the Sword.
Sacha Baron Cohen was offered $7 million for the role of Kulan Gath but he turned it down. The character originally appeared in the Conan the Barbarian comics as a foe of Conan by Marvel and the Red Sonja comics by Dynamite Entertainment when they got a hold of Red Sonja.