Thousands of years have passed, and still Jack does not age. Having become a wandering vagrant, he solves the problems of the week like back in the first 4 seasons.
Then he hears news of a new darkness in the land. Jack arrives to find that the darkness that Aku spawned from had survived its encounter with the gods and came to collect its pieces. It then rends a portal in time and pulls Aku from the past before he was killed and brings him to the present. This time fuckery undoes season 5, but Ashi is around.
At first Aku likes having someone his power level to work with against Jack, but the Darkness has no sense of humor and intends to absorb him. Ashi tries to use her time powers again but the Darkness removes her powers.
Jack eventually teams up with Aku when they find out the Darkness intends to destroy everything; Aku likes everything, he just wants to rule it. Aku betrays Jack when he sees a chance to destroy him once and for all, but Jack is actually carrying a fake sword. Ashi, aided by the Scottsman, defeat the darkness by combining the sword with Celtic Runes.
It is at this point that Aku starts to grow; it turns out that Aku was only so powerful in the previous seasons because the darkness was alive then too, holding back his power. Aku attempts to infuse the entire Universe with his essence, but finds Ashi is immune because the part of her soul that would be possessed by Aku was ripped out when the Darkness took her powers. She stabs the item Jack was carrying on his person the whole time with the sword; the original arrow that spawned Aku long ago. This destroys Aku and everyone lives in a distopian future and Jack gets his waifu.