First up is a comprehensive and intriguing perspective on the poe currency design philosophy behind the Undying. Developers felt that the Undying had become a "relatively unremarkable monster type" and wished to do something more to help it stand out from the match. To this end, the team went through a collection of preparation stages to create something unique for the race throughout the game.
Our aim with the player's first experience of Act Three has been that the first place would be quite silent, with no monsters to fight outside. Upon entering the shade, though, players could be billed by mortal City Stalkers and might have to escape to the safety of the sunlight. Their expertise with the remainder of the Act will involve a match of caring very much which regions were shaded, while sometimes being made to step from their light to travel between buildings. Because of their early encounters with the City Stalkers, they would fear every moment spent out of sunlight, as it is one step away from being swarmed from the mysterious monsters in Path of Exile. We planned for City Stalkers to have immense life regeneration, preventing players from making reasonable progress past them.
Gems
Next upon the design docket is a discussion of the design and development of Gems. Most importantly, the thought process behind the most recently-introduced stone is researched. As you may recall, nine new jewels were inserted "including four wrought stone that concentrated on Necromancy", one of the trademark characteristics in War for the Atlas. Senior Game Designer Rory discusses moving from an overall discussion of abilities that would be inserted and how gems could fit into that design structure. For instance, taking the base ability of Detonate Dead and changing it with the accession of a stone was something that the team. Of course, some abilities worked better than others in this regard, but, as Rory states, all that is part and parcel of sport development. .
We had also prototyped a ability codenamed "Infernal Sweep" which was a fiery area of effect sweep attack that burst nearby corpses. Early testing showed it felt awful to use, needing to swing twice to first kill several enemies then again to detonate their own bodies for bigger area damage. The skill was spectacular, but didn't play well unless it had been boosted in both starting place and harm to the point where it invalidated the functions of Sweep and poe marketplace Infernal Blow. This skill went back to the drawing board and we'll probably see it in future with additional mechanics or without the reliance on corpses.
The article further goes into specifics about Support Stone and how layout iterates from concept to completed project. You can check it out here.