Love to see new Mythic Bastionland myths, especially one with a fantastic opening poem! I also really like the writing down the page margins, they draw me into themes of the Verse. The inclusion of the tulpa and alzabo is really cool!
The Malign Swamp may have a typo in “Whenever a Knight makes a Save at this location, they roll 2d20, and take the lower of the two results.” Is it possible this was meant to say “greater”, to increase the probability that saves are failed?