Parmandur
Book-Friend
I think this book has some great ideas, I love the whole piety mechanic and supernatural gifts and can imagine using these in other settings. Mythic monsters are awesome and a great way to include boss fight phases in the game.
Since you've mentioned anvilwrought this brings up one of the things I find annoying about the 5e books, they don't always include artwork for a monster. I have absolutely no idea what a burnished hart is because it has no artwork and the description doesn't provide much detail. I guess it might be some sort of deer or ram but I honestly have no idea.
They have the art resource, probably wasn't room for it: