The birth and early life of Karnilla were shrouded in mystery. All that was really known is that she had long lived in Nornheim, the Asgardian province where the Three Norns dwelt, the goddesses of destiny, and she became one of the most accomplished practitioners of the arts of sorcery in the Asgardian dimension. Although Nornheim lay on the Asgardian landmass within Asgard's own dimension, and although Karnilla was apparently of the Asgardian race, she did not consider herself to be subject to Asgardian rule. Rather, Nornheim was an independent country, of which Karnilla was absolute monarch.
Karnilla lived opulently in the ancient fastness of Nornkeep, attended by her servant, the aged minor sorceress Haag, and troops of guardian demons. Although there were many demons among her subjects, Karnilla's realm was also populated by humanoid immortals, just as was Asgard proper. Karnilla's relationship to the goddesses of destiny was unknown, although they were clearly beyond her jurisdiction.
In the past, Karnilla often proved to be an enemy of Asgard, and acted in alliance with the Asgardian god of evil, Loki. However, Karnilla was long in love with Asgard's noblest warrior, Balder the Brave. On Balder's request (on behalf of Asgard's monarch Odin), Karnilla allied herself and her army with Asgard in the war with Surtur. After years of resisting Karnilla's advances, Balder finally became Karnilla's lover. Some of Balder's Asgardian friends disapproved of the relationship, regarding Karnilla as a figure of evil. Karnilla regarded herself, however, as a harsh but just ruler.
Balder was called away from Nornkeep to join the Asgardian prince Thor in his mission to rescue souls of Earth human beings from Hel. During Balder's absence, Karnilla was taken prisoner by the Frost Giants of Jotunheim, who were preparing an invasion of Asgard and first intended to eliminate Asgard's allies. The Frost Giants also transformed Karnilla's subjects into stone. On his return, Balder set out to rescue Karnilla, and was ultimately met with success thanks to the help of Tiwaz. Karnilla and the people of Nornheim then helped stop Hela's invasion of Asgard with the Valkyries. Karnilla continued to both antagonize and aid Asgard. She would often aid it against a mutual threat, but continued to seek Balder's affection or seek the lives of Thor and Odin to try and make Balder king.
Since the return of the Asgardians after Ragnarok she at some point made an alliance with the troll king Gerridur. She enchanted a charm that disguised Ulik as an Asgardian Tanarus and altered everyone's memories so Tanarus was always the champion of Asgard and everyone would forget Thor. Ulik was to use his position to assassinate the All-Mothers of Asgard at the appointed time to throw Asgard into chaos for a troll attack. Karnilla formed a triad of outcasts witches called the Weird Sisters along with Kelda to spy on Asgardia and aid Ulik. Despite the spell, Heimdall eventually saw through the spell and destroyed the charm just as the trolls attacked. Karnilla joined in the battle in a desperate attempt to turn the tide. She battled and was quickly defeated by Freyja. Freyja then sealed Karnilla and Gerridur within the skull of Ulik.
Karnilla was part of the list of women Odinson suspected to be the new Thor. She was one of the many women warriors (Asgardians and Avengers alike) Odinson and Freyja brought to help Thor in her fight against Cul Borson, as he commanded the Destroyer armor under Odin's order. They fought him off until Odin aborted Cul's mission.
Karnilla requested Odinson to help defend Nornkeep from a massive army sent by Malekith to kill the Norns and prevent them from determining their destinies. When the attack occurred, Karnialla had the Norns evacuate while she remained behind to defend the Nornkeep. She immobilized many of the attackers by taking control of their destinies. However, the army was able to sever Nornkeep's connections to the roots of Yggdrasil, causing the massive structure to collapse. Karnilla was caught in the collapse and mortally wounded. Before dying, Karnilla told Odinson that she had witnessed visions of many things, including the death of the new Thor, Jane Foster. She tried to warn Odinson to protect Jane but died before she could.
Karnilla's spirit was sent to Hel, the realm of the dead, where she reunited with Balder, who had become the realm's new ruler. Moments afterwards, Hel was attacked by an army of Fire Demons from Muspelheim on behalf of their queen Sindr, one of Malekith's allies. Balder's forces were overpowered, but he received aid in the form of Thor and Loki, who teleported to Niffleheim to put an end to Malekith's plans. Tyr tricked Balder and his allies into liberating Hela, who was being returned to Hel as a prisoner of the Queen of Cinders. Loki convinced Balder and Hela to marry despite their enmity for tactical purposes to unite the realm against Sindr. Karnilla was displeased with this development, but wished the best for the realm. Before the marriage could be sealed, Sindr's forces attacked. The Queen of Cinders was defeated by Thor, who had also recruited the Einherjar and the Valkyries to fight for Niffleheim. After Sindr and her forces retreated back to Muspelheim, Hela refused to continue the ceremony, arguing that Balder never got to say his vows. Having stolen Balder's wedding ring from him, Karnilla slid it in her finger and said "I do" in his place, sealing the marriage to spare Balder of the sacrifice. Karnilla subsequently requested Loki's aid in order to return Balder to the land of the living.
Despite Hela's infidelity with Thanos,[ Karnilla adjusted to co-ruling the Asgardian afterlife. When Malekith was returned to Hel following his death in the War of the Realms, Karnilla devised a tortuous punishment for him that left even Hela impressed: excising the part of his soul embodying his child-self and forcing him to watch as it played happily with the souls of his bog tiger and hounds - which were reverted to a cub and puppies.
