A 'mark of chaos' in an MLP fanfic could potentially revolve around a new concept or power that disrupts the normal harmony of Equestria. It could be a mark that appears on a pony, endowing them with chaotic abilities. This might lead to various storylines, like the pony trying to control these powers or others being afraid of them. Maybe it's a fan - created story exploring what would happen if chaos was introduced in a new way to the MLP world.
Tian Xing Jiu Ge was the short form of Tian Xing Jiu Ge and Tian Xing Jian. It was not a sister chapter or a brother chapter. Tian Xingjian was the second part of Tian Xing Jiu Ge, and Tian Xing Jiu Ge was the first part of Tian Xingjian.
You can try looking on popular fanfic websites like Fimfiction. It has a large collection of My Little Pony fanfics, and there's a good chance you might find a 'mark of chaos' one there.
I don't really know who Mark Brown is in this context. He could be an author, a psychologist, or someone who has a particular perspective on life as a story.
One way to make songs longer in a story is by including additional musical interludes or expanding on the instrumental parts. Also, you could tell a more detailed story within the lyrics, which would naturally increase the song's length.
To make story songs longer, try incorporating additional characters or plot twists. This gives you more to sing about. Another option is to extend the instrumental sections and build up the atmosphere. Also, repeating certain key lines or motifs can add length without it feeling forced.
You can add more verses or extend the chorus to make songs longer in stories. Also, describing the background or the emotions related to the song in detail can add length.