Abyssal, the energy of unlife. Where Aura is found within all living creatures Abyssal resides in the space between universes and planes, where life is devoid. It can seep into reality where life is fleeting, such as the sites of genocide or sterilization.
Abyssal cannot be used by living creatures in most instances, leading to severe recoil damage on those who attempt to harness it recklessly. Those devoid of life such as the undead or robotic beings may find more success but will require some way to gain a stable source of it.
The primary users of Abyssal are Harbingers. Either through pacts with the Elder Gods, who are beyond life, or by being outright created by them they are able to funnel Abyssal energy from between worlds, giving them a virtually infinite well to draw from. Abyssal often serves as the spiritual glue that holds Harbingers together, which means getting cut off from their source can prove dangerous.
Abyssal's applications match Aura's, though are wilder and more destructive. Even defensive Abyssal applications will hurt to the touch or somehow destroy. Pound-for-pound Abyssal is stronger than Aura in terms of damage but struggles to match Aura in support or utility.