I won't spoil you the juicy details, but the Fuuzetsu is created by Leanan-sidhe, who is not yet known by that name. She's also known as the Spiral Organ and Lamies, the old guy that Margery Daw wants to eliminate in Season 1.
The Great War is the war told in Eternal Song. It's a unique experience for the humans involved, especially when they have to face a Crimson Lord strong enough to carve a canyon.
UPDATE: Detexting the two-page spread. Here's how I'm doing right now.