The History series Vikings depicts a community of Viking warriors, including the famous Norse warrior Ragnar Lothbrok and his first wife, the Viking warrior Lagertha. The character of Lagertha is based on a real Viking shieldmaiden - born sometime around 795 CE - who really was married to Ragnar. Her story is even more amazing than depicted in the TV show - she sicced a bear on Ragnar while he courted her, almost single-handedly won a civil war, and murdered her husband so she could rule a kingdom on her own. Although historians and archeologists can't seem to agree on whether or not Lagertha (and shieldmaidens in general) existed, a 12th-century Danish historian named Saxo Grammaticus, or Saxo the Learned, includes Lagertha in his history of Norse warriors and heroes.
After fighting with Lagertha to defeat King Frø - the king of Sweden who had invaded Norway - Ragnar Lothbrok became enthralled by the amazon-like woman who had led a charge of her peers into battle and succeeded. According to Saxo Grammaticus, Ragnar even credited Lagertha with victory in the battle. Her combat skills led Ragnar to begin courting her.
Lagertha decided to have some fun with her suitor and feigned interest to see how far he would go. When she realized he wouldn't give up, she invited him over, where her giant dog and pet bear waited for him. Ragnar, somehow, wasn't fazed by this and managed to kill both animals. With Ragnar having passed this trial, the two were married.
Along with being incredibly strong and brave, Lagertha had a good heart. When Ragnar Lothbrok, who divorced Lagertha in 817 CE, found himself embroiled in a civil war, she decided to help him out with her fleet of nearly 200 ships, going into battle with him after he was forced to beg for her help. She led a counterattack from behind, and quashed the entire war practically on her own.
Ladgerda, who had a matchless spirit though a delicate frame, covered by her splendid bravery the inclination of the soldiers to waver. For she made a sally about, and flew round to the rear of the enemy, taking them unawares, and thus turned the panic of her friends into the camp of the enemy.
Saxo Grammaticus claimed Lagertha helped Ragnar because she was still in love with him. Following the civil war, the two returned to Norway together and were remarried.
After returning from the civil war, Lagertha grew tired of sharing a kingdom with her husband and decided to kill him. In one of the best descriptions of Lagertha given, Saxo Grammaticus wrote that Lagertha killed her husband because she "thought it pleasanter to rule without her husband than to share the throne with him." The issue is that history is a bit fuzzy on which husband Lagertha actually killed.
Some believe it was Ragnar, who went back with her to Norway for a bit, and it seemed like the Viking warrior would get her happy ending with her ex-husband whom, according to Grammaticus, she still loved. However, while in Norway, the pair got into a vicious fight, so Lagertha decided to just kill him instead, stabbing him in the heat of the moment with a spearhead hidden in her gown.
Despite numerous sources stating she did in fact kill Ragnar, an in-depth reading of Grammaticus's work implies that Lagertha actually did not stab Ragnar, but her second husband, usurping his title and kingdom, just like in the TV show Vikings.
Lagertha is mythological in her legacy, and she is commonly associated with Norse spirits, the Valkyries. She officially has the status of a warrior goddess, as the Valkyries were female spirits who helped the god Odin decide the fates of battles. It makes sense that Lagertha - a notable female warrior of her day with the small frame of a woman but the bravery of a man (at least according to Saxo Grammaticus) - was associated with these warrior spirits. Some scholars even believe Grammaticus based his account of Lagertha on stories of a goddess named Thorgerd, furthering Lagertha's legend as a warrior goddess.