Before Ashi can kill the samurai, many of Jack's allies arrive and assault Aku and his fortress. After they free Jack, Aku overpowers them, and Ashi prevents him from recovering his sword. ... Jack finally kills Aku, freeing his family and undoing Aku's future.

What happened to Aku in Samurai Jack?

That time, however, Jack used his sword to destroy Aku once and for all: Jack plunged his sword with Aku still inside into the ground, which then caused Aku's Castle to explode, took Aku with it, which prevented the rise of Aku and spared both the universe and the future from his tyranny.

Is Aku really dead?

And, thankfully, the fifth season finale did not disappoint. The last episode of Samurai Jack kicked off with a bang, and it ended with one long-awaited death. Yes, that's right. After years of dreaming back going back to the past, Jack was able to do so and finally killed Aku.