After you kill faceless void in a left side of a map, it opens portal to arena where 4 phantoms constantly spawn. They drop some crystalls called "increased damage", "increased lifesteal", etc. I wonder what are they for? You can't use them or put them into forge. Or maybe they work like runes in diablo 2?
Also there are 7 terrorblades in a centre of a map. They drop 7 books, which you can compose into 1 book, and 7 souls. I couldn't find use of any of these items. Maybe someone knows?