WARNING: The following contains spoilers for Kingdom: Ashin of the North, now streaming on Netflix.
One of the things fans were hoping for in Kingdom: Ashin of the North was to see how the titular archer tied into the main series. It was clear she was helping seed the resurrection flower across the Korean border, giving rise to zombie animals and hordes of the undead that the Joseon government was trying to keep secret, but how it connected to the main story was still a mystery. Well, come Ashin of the North, a dark truth comes to light about Ashin's actions and how she was eyeing the destruction of the crown by infiltrating the king's inner circle.
In the post-credits scene of Ashin of the Nort, Ashin, older and more calculating, is seen after she takes out the vile commander, Chi-rok, and his Joseon military camp for using her village as a pawn to placate the Jurchens. She's meeting Lee Seung-hui, the King's physician at Joseon Border, Uiju, to educate him on the resurrection flower, which she used to raise dead soldiers in Chi-rok's camp.
A flashback then shows Ashin and the old doctor before the fabled "Battle of Unpo Wetland," a mysterious war that was chronicled in the first season. It's revered by many because 500 Korean soldiers stood tall and defeated an army of 30,000 Japanese invaders. And Lee-chang and Seo-bi (Lee Seung-hui's protege) would discover from his journals and research that the now-dead physician helped the evil General Cho by offering them the flower as a weapon to create more soldiers.
The ailing villagers of Sumang were used as guinea pigs, transformed into murderous zombies through the resurrection plant, and it's now clear it was Ashin who taught the physician the trick. Apart from bringing him the plant and flower, she detailed how to sew it into the foreheads of dead soldiers and stitch it back up so they would rise up as zombies. That's what she did at Chi-rok's base, and from this revelation, the Korean army would use the same tactic years later. After the war, they'd kill the zombies and bury the secret, never realizing Ashin was the real puppet master.
It was a two-fold plan because she knew, at some point, it would be used by Cho or even by the royal court to save a dying king. And given how stealthy Ashin is, she could've poisoned Lee-chang's dad to kickstart everything, especially since Season 1 focused on Cho using a zombified king to pass the throne to his daughter's son after a sneaky marriage. This was all part of Cho's plan to banish Lee-chang and take the crown for himself after the king's death, so Ashin knew exactly what she was doing, seeding discord with the plant because she knew men of power would misuse it.
The revelation also brings the Cho clan's story full-circle because they killed the Jurchens, which led to Chi-rok trying to cover for them. He used Ashin's village as scapegoats, with the Jurchens slaughtering them in revenge and putting Ashin on her dark path. In that sense, the Cho clan unknowingly became the catalyst for a new zombie problem years later. But with Lee-chang's young brother as king and showing signs of infection, Ashin's plan is still in play, and Season 3 is set to detail what other chaos she's got planned.
To see how Ashin's plan came to fruition, Kingdom: Ashin of the North is streaming now on Netflix.