Madam Slay's past is mostly unrevealed. She hooked up with Erik Killmonger, eventually becoming his lover. She was absent during his direct attacks on the Wakandan royalty.
After Killmonger's death, Madam Slay located the broken body of her former lover and brought it to a temple, where she preserved it with secret herbs.
Months after Killmonger's death, she led Mute and her leopards in the slaughter of several Wakandan leopard hunters. When the Black Panther and W'Kabi came to investigate, she directed Mute to capture them as well. She had T'Challa at her mercy, but rather than kill him outright, she tied him to two of her leopards, who drug him across the harsh terrain. T'Challa eventually took advantage of the situation, mounted the leopards and rode them back to her. She stabbed T'Challa in the back as he was struggling with Mute, but W'Kabi shot out her footing and she fell, knocking herself out on a rock.
Madam Slay continued to keep watch over Killmonger, until one night, as she slept, the Mandarin entered the cavern she was using, and took Killmonger's body. Apparently, the power of the Mandarin's rings was able to amplify the magic of the Resurrection Altar, restoring Killmonger to life. Three days after his disappearance, Killmonger returned to her and revealed his plot to return Wakanda to its ancient ways.
Madam Slay assisted Killmonger in a plot to assassinate T'Challa, take over the role of leader of Wakanda, and lead the country to war against other nations. While Tony Stark visited Wakanda, she used her feminine wiles to put Jim Rhodes off guard, embracing him and then stabbing him with a heavy dose of sedative drug--Rhodes then became their prisoner. The murder of the Panther apparently succeeded and Killmonger framed the "foreign devils," convincing his countrymen that he could lead them to vengeance. They hooked a cyclotron device to the sacred mound of Vibranium, hoping to convert it into its other isotope, more commonly known as anti-metal. The enormous mass of anti-metal would set off a chain reaction that would convert all of the metal of the world into a virtually useless liquid form. However, T'Challa revealed he had faked his own death with a Life Model Decoy, and he defeated Killmonger while Iron Man deactivated the cyclotron and prevented the mass conversion. With Killmonger's defeat, the Mandarin recalled his ring, and Killmonger reverted to an inanimate skeleton.
Short of calling in Killmonger's troops when the heroes arrived, Madam Slay did little to nothing to assist in the final battle. With Killmonger's second death, she was emotionally crushed and gave up once again.