Here's how Samurai Jack's demonic villain Aku is finally defeated. The Genndy Tartakovsky created Samurai Jack follows the title character, who is raised from a young age to slay the shapeshifting demon Aku in feudal Japan. Just as he's about to deliver the killing blow, Aku opens a portal that sends him to the future, where the demon's rule is absolute. The original Samurai Jack episodes thus focused on the wandering samurai as he battled Aku's minions such as Demongo, meets new allies and searches for a way home.
Samurai Jack's unique mix of great action and animation with likable heroes (and villains) made it a cult favorite. Sadly, devotees of the show had a long wait between seasons 4 and 5, with Tartakovsky becoming busy on other projects. There was always talk of another series or even a movie, but it wasn't until Adult Swim picked up the show that Samurai Jack season 5 emerged over a decade later in 2017. This series picks up half a century into the future from season 4, where the ageless Jack is still roaming the land and the story was designed as the final chapter of Jack's story.
Samurai Jack's ending was quite heartbreaking for some, as it initially appeared to grant Jack a happy ever after - before snatching it away. Season 5 as least ends with the unambiguous defeat of Aku, which has several shades of irony to it. The final season saw a new Aku voice actor step into the role, with Greg Baldwin taking over from the late, great Mako. This Samurai Jack season also introduced Ashi, a member of the cult Daughters of Aku, who were born from Aku's essence and raised to slay Jack. After the latter kills her sisters in self-defense and she learns what a monster Aku is, they team up and later fall in love. Ashi would also be instrumental in Aku's destruction.
Samurai Jack's finale has the title samurai captured, with Aku having controlled Ashi like a puppet and brought out the evil within her. The demon wants to add one final humiliation by having his daughter kill Jack herself in front of the world. Thankfully, while Aku was distracted fighting off an attack by Jack's allies, the samurai is able to bring Ashi back to herself. She still has Aku's powers and creates a portal to the past that both she and Jack jump through. They then travel back to the moment Aku banished Jack at the very start of the series, appearing to a shocked Aku mere seconds after Jack was first sent through the portal.
Jack wastes no time hacking and slicing the weakened demon, and traps him in his sword. The furious samurai then stabbed the sword into the ground, finally killing the demon and destroying his castle as Jack and Ashi fled. Samurai Jack's defeat of Aku was immensely satisfying, and it was an excellent touch to bring it right back to the very beginning.