Originally Posted by
NMX-
When youre in elf form you have two resources: Angst and Fury. Angst is just a normal energy bar for elf abilities that we're used to today.
You build up fury for each attack you do. Once you reach 100 fury, all of a sudden you turn into your demon form, releasing the demon from within. You are able to walk away from your elf form, which is phase-shifted and immune to all damage and abilities. Your job as your demon form is to stay in demon form for as long as possible before your elf form takes "control" again. Your health pool quadruples, but it begins to decrease in size as you lose control. You return back to your elf form if (1) your demon form reaches 0 health, or (2) you run out of fury (30-40s, time-based, but demonic abilities and damage extend it longer, though).
To me this would feel true to the Demon Hunter fantasy we see from boss fights like Leotheras and Illidan.