Avatar: The Last Airbender and The Legend of Korra establish a saga that spans centuries, outlining the continuous cycle of reincarnation of one Avatar, the sole-bender who can master all four elements: fire, water, earth and air. However, while the TV shows focused on two incredible heroes -- Aang and Korra, respectfully -- there are other, previous incarnations of the Avatar in the past.
While Aang and Korra are powerful, the word "deadly" isn't one you'd really apply to either of them. They're loving and gentle people. However, it is a word you'd use to describe Kyoshi, an Earth Kingdom Avatar with a long, storied history of terrifying her enemies into submission with her incredible power.
Kyoshi, Origins
At the time of Kyoshi's birth three hundred years before the Air Nomad genocide, the Earth Kingdom had been destabilized by Xu Ping An's Yellow Neck Uprising, where multiple groups of bandits ganged together to demolish all law-abiding officials. Kyoshi's parents, Jesa and Hark, were the leader of one of the Earth Kingdom's most notorious crime syndicates: the Flying Opera Company. They were continuously on the move, laying low to keep Kyoshi safe. They left her in the care of the villagers of Yokoya Port, who cast her out and left her fending for herself. She blamed her criminal parents for her lot in life. As she grew taller than most men on a diet consisting mostly of garbage, she grew to resent all crime.
This was until she encountered bending masters Jianzhu and Kelsang by chance. Jianzhu had recently stopped the Yellow Neck Uprising and spent the last seven years looking for the next Avatar. As it turned out, their journey ended upon meeting a boy named Yun, who they mistakenly believed to be the Avatar. Kelsang, an air nomad, took Kyoshi under his wing, until he realized, too late, that perhaps this starved girl was the real Avatar.
Kyoshi would align herself with Yun, choosing not to reveal that this boy wasn't the real Avatar in fear that doing so might invalidate a treaty being signed with a group of pirates known as the Fifth Nation. This went south when the pirates attacked and Kyoshi managed to stop them. Jianzhu brought Kyoshi and Yun to a remote place where he summoned a spirit to determine which of the two teens were the Avatar. Learning the truth, Jianzhu let the spirit seemingly devour Yun alive while preventing Kyoshi from saving him.
This was a lot for a 16-year-old to take in, and things would only get worse from there. Both Jianzhu and Kelsang fought over who was better suited to protect and help Kyoshi, and Jianzhu killed Kelsang before Kyoshi's eyes. This traumatic moment forced Kyoshi into the Avatar State, releasing enough power to literally demolish an entire village -- though Jianzhu somehow survived.
Kyoshi would go on to train in and master the four elements, before eventually confronting and matching Jianzhu in power -- though she didn't kill him. That honor belonged to Yun, who hadn't actually died at Jianzhu's hands.
The Continent-Breaker
Kyoshi would go on to form the elite battalion known as the Kyoshi Warriors. Initially created as a task force designed to discourage sexual discrimination and assault of women, they would later prove an instrumental fighting force in conflicts for centuries to come.
Years later, Chin tried to conquer the Earth Kingdom. His campaign against the unpopular Earth King seemed impossible to stop, until Chin ran into Kyoshi. Chin expected a fight, but he didn't expect Kyoshi to literally rip a peninsula off the rest of the continent, sending it out into sea to crumble beneath the conqueror's feet. Kyoshi did this by mixing multiple elements under her command: heating up the ground, creating a cataclysmic earthquake, before generating hurricane winds to carry the splintered square-miles of land off. Chin did not survive the assault.
She singlehandedly defeated the Earth King's entire army and "persuaded" him to restructure his government, creating a democracy for the peasants who lived there. This created the Dai Li, a force that would eventually grow corrupt in Aang's lifetime. Warlords and kings bowed to her will -- though, sadly, her promotion of LGBTQA rights never went very far. Kyoshi, being bisexual, tried to fight for acceptance, but despite being able to change government structures, could never persuade people to open their hearts to the issue.
Kyoshi lived to be 230 years old, becoming the oldest Avatar ever. In a sense, she lived on long after, consulting future Avatars Aang and Korra from beyond the grave. The two did not take her more brutal advice. Aang in particular refused to follow Kyoshi's advice to kill Fire Lord Ozai.