Aetherium

Strange Miasma

Gods can't really die, they can only be shattered, their sentience destroyed, and their energy left to decay. These lingering energies are called Aether, and there are various types, each divine corpse leaving behind a different type of aether. Most of them stick to the location where their god's corpse struck the earth. However, others have drifted across the world, with varying consequences.
Most of the dead gods in the world died during the war of the wayfarers.

Contagion

One of these Aetherium, the dead godpreviously known as Balghun, has been rotting in the earth since the war of the wayfarers, its festering essence piggybacking on the energies of Velen and Forsaken alike. This has caused the phenomenon known as the contagion.
It is possible to detect this contagion, the easiest way being by casting concentration. However, this won't help if the energy used to cast it is also contaminated. There are also magical mechanisms such as detectors and purifiers that are used to verify the absence of the contagion, and resolve it if necessary.
When a mage casts magic using contaminated energy, they will be hit with a strange feeling, the smell of sulfur, and the taste of rotten fish filling one's lungs, and a surge of power and adrenalin. The contagion, being an energy itself, will boost any spells cast, increasing their strength, as well as increasing the ease of energy flow, like a slick lubricant.
Some people use the contagion on purpose, seeking it out to increase their power, to feel the adrenalin, as those who can stomach the vile smell and taste will be able to cast spells of great power. Those who become addicted to this power and adrenalin are called taint sniffers.
When a mage makes a habit out of doing this, either due to addiction or otherwise, their body will adapt to the contagion, the smell or taste will become faint, the body will become much more attuned to it, but over time, the ability to cast without it becomes neutered. In addition, it will start to affect the brain, dulling the senses, while at the same time, making the individual more irritable, over time, their mental state will decline even further, they will start to see and hear things that aren't there, they will start to suffer from delusions, aggression, and over time, complete insanity.

Lumina

In the far south, deep within The Shadowlands, there exists a force of nature, called the Lumina. It is hidden in shade, and from it emanates a strange pulsating heat. It warms the south enough to be of a comfortable temperature, and even tropically warm as you approach it. Its exact workings are a mystery to the people who are not from this region, and the inhabitants of the shadowlands are not too keen on people snooping around their dead deity. They were shaped by it, influenced to a point where they can barely be seen as human anymore. This is something they are quite proud of.
Besides turning what would otherwise be a frozen wasteland into a fertile archipelago, and its effects on the people that live here, it also influences the sky, the weather, and anything else that gets to within its sphere of influence. It has painted the aurora with strange colors, it turns clouds a faint red, and it causes frequent thunderstorms.

Energy is eternal, it can neither be created nor destroyed, it can only be laid dormant, shut from existence, or shattered into pieces.

Related Articles:

The Sleeping Dead

Gods die, but the sacred energies of the source cannot be undone, even if their vessel has been destroyed, they are capable of simple taking a new shape, their shattered essence can rejoin together, even along with that of other fallen deities, and awaken as something new.