The Rite of Asdeaon

Asdeaon's Birth

Four centuries prior to the current age the nations of Ral, the first continents, experienced a war never to be repeated. Fighting amongst themselves the gods that shaped the world battled over their greatest and most heinous creations, mortals. The gifts given to the mortals were too powerful, as such a mortal rose amongst the ranks of the divine, shattering the exclusivity the divine previously shared. This act brought upon by a deity named the Harbinger of War, brought upon the Divinity Wars. For three decades the gods fought, battling each other and the mortals who worshipped their enemies, siblings against siblings, loved ones against loved ones, until all shreds of divinity were banished from Ral, saving mortals and the world the divine created.
During the three decades of combat, one battle would scar the country of Leanegras, a forest of immense size took part as the background for a melee between the gods. A being of rage battling a deity of light, two gods of colossal power whose strides made mountains. Their battle was legendary as the entity of light nearly struck down her enemy, an attack that pierced the very earth for miles, wounding the rage filled god. The melee ceased after this moment, fleeing as he bleed his divine blood into the newly made canyon the god of rage vanished, their combat forever scaring the world. From the blood of the wounded god, a new entity was born. One of malice, vile intent and corruption. After the banishing of all other divine entities, this new entity, a power...no, a new demi god born on the surface of Ral surfaced. A being who named itself Asdeaon. The forest of Asdeaon, more commonly known as the Crimson Forest is a region tainted by the corrupting radiance of Asdeaon, the new godling. Its magic seeps into the ground, warping the natural landscape into a mockery of nature with a ceaseless desire to corrupt the entirety of Ral.

Asdeaon's Rite

The Rite of Asdeaon is a representation of its will to corrupt the entirety of Ral, starting with the nation of Leanegras. Every 200 years, Asdeaon selects champions, entities that will enact its will to prepare the rest of Ral for the continued spread of the Crimson Forest, Asdeaon's demonic domain. As such, every 200 years, a new collection of Heralds of Asdeaon are found, created, and in many cases sequestered from their fate in the heavens and hells. These beings are given power from Asdeaon to prepare for the forest's growth.
In the history of Ral, the Rite has been invoked once in the past conjuring a series of demons that rival the power of dragons for Asdeaon's machinations. 200 years prior to the current age in Ral, this group of demons was vanquished by a union of elven-kind and dwarven-kind, uniting for the first time in centuries.
As of 427 Post Divinity War in Ral's history, the Rite has been invoked once more, conjuring demons, tainted celestials, and nightmarish creatures to conquer the entirety of Ral, not just the swath of land currently controlled by the ever ambitious Asdeaon.

Manifestation

The very presence of the Heralds, the entities conjured by the Rite of Asdeaon have numerous effects within Ral, most evident near the Crimson Forest:

Aesthetics:

There is an ever present chill within 30 miles of the Crimson Forest, enough to be noticed but not enough to create snow or ice.

The sun does not shine upon the Crimson Forest, clouds seem to block the radiance of the sun above, casting the forest in permanent darkness. 

Non-magical fires are weakened in the forest, as such they provide little relief from the region's chill

Mechanical effects (up to DM interpretation)

Demons and Undead within the Forest's bounds are enraged. While within one mile of one of the Heralds they are harder to kill and gain mechanical advantages deemed appropriate by the DM. 

Those with divine powers and abilities such as clerics or other such player character classes feel the presence of their god waning upon entering the Forest. This can cause panic, mechanical disadvantage, any number of effects. 

If a creature is to fall to unconsciousness due to combat wounds, they have disadvantage on any death saving throw made within the Crimson Forest.

Localization

All effects of Asdeaon's Rite directly impact the Crimson Forest, the home of Asdeaon and its Heralds. Pockets of Asdeaon's corruption have seeped through the bounds of its forest, infecting other parts of Ral. These micro-environments are subject to the same effects the true Crimson Forest is subject to. 
Additionally, these smaller pockets of Asdeaon's Radiance are perfect introductions for playgroups into the Crimson Forest, Asdeaon as an entity, and the Heralds as a small adventure or a full campaign arc.

Type

Metaphysical, Demonic