Histuzen was a story about a group of assassins. They were going to kill a noble's son, but Histuzen has a change of heart at the last minute and it fucks up the whole mission, leading to the downfall of the group. They quickly blame Histuzen, calling him a traitor. "Histuzen" also means "fate" so you can also read this as "Fate was the traitor." Storywise, I really didn't go much farther than that.

I like to think that in Maikai, humans, who are very rare, sort of have the same place in society similar to fantasy creatures that have been made up for tv shows, so it's not uncommon to see a few movies in Maikai with "humans" in them.