Overview
Nether Energy is one of the 3 types of energy within the Tyren setting. It is nothingness incarnate, defying physical description.

In-depth Explanation
Nether Energy is the antithesis of Life energy, and when the two meet, they cancel each other out. This isn’t like matter-antimatter reactions, which produce insane amounts of energy and explosions. No, this is a simple cancellation of forces, albeit with a very painful result for the poor fool who was just hit with Nether Energy.

Describing this feeling from character’s point of view is nearly impossible, simply because those who survive such attacks in any appreciable amount are incoherent about the experience. It’s as if their very mind doesn’t want to remember, and it doesn’t, because Nether Energy is from outside Creation, and created beings simply weren’t made with the capacity to understand and relate the cancellation of their life-force.¹

However, the physical effects are easy to describe.

Effects
The effects of Nether energy on the body is rather simple. Visibly, the victim begins to turn a shade of gray, roughly as if the color was being drained from their skin. While the kinetic energy of a blast of Nether energy would be roughly akin to a punch in the gut, the energy itself does nothing to the physical makeup of the target’s body.²

Under the hood, however, there is a much more noticeable effect. Because Life energy is the lifespan of a character, the effect of Nether energy is very much life-threatening. When the two cancel themselves out, you are left with a shorter lifespan, which can cause death in a timely manner.

The lack of direct physical damage from Nether energy isn’t a cause for PCs to rejoice, however. The forceful removal/cancellation of life energy causes exhaustion, with the result that a character’s reaction speed and fighting capabilities go down the drain very quickly, leaving them unable to avoid further harm.

Resisting the Effects
While no one can resist the effects of Nether energy for extended periods of time, a character with a strong life-force can resist it’s effects for a short period. Allow characters who encounter Nether energy, whether by spell or ambient, to make some sort of constitution check, with a success meaning that they have fought off the effects of the Nether energy.

For spells, this would be handled by the usual spell-resistance rules of whatever system you happen to be using. For ambient Nether energy, a periodic check every so often would be fine, with an increasing difficulty to each check without a reprieve from the Nether energy. After all, a person can fight it off for a while, but eventually even the strongest would succumb to it.

¹If incorporating this into horror campaigns, Nether Energy can very much be used as you would use Other-Worldly beings; the concept of incomprehensibility is the same.

²This can be used to added effect in a horror game, because of the lack of physical damage.

GM: "The blast of dark energy hits your ally, causing no visible damage, but he crumples to the floor anyways, all the color drained from his skin."
Players: "What?! Let’s get outta here!"