Shadowspill is an area of Shadow corrupted land in Canstice spreading from Calori after the events of the battle of the Shadow Bridge.
Also affects Labylithea and the labyrinth
How Demon Portals work (MToF)
In MToF (Mordenkainen's Tome of Foes), there is an explanation for how demon portals work with four distinct phases. But whenever I try to look it up, I can never find a reference and I have to go sifting through the book again. So I thought I'd post it here, with less verbose definitions of each phase.
Phase One - Initial Infection
A demon portal takes time to develop. Phase one involves any sort of demonic invasion. The area around these demon(s) becomes tainted: plants twist, water becomes murky (or even poisonous), and the weather can become extreme (snowstorm in a desert, extreme heat, hail, etc.).
The infection ends if the demon(s) are slain, venture away, or return home. The physical twisting of the world subsides over several months. But if the demon(s) remain in one spot undisturbed, the infection will advance to phase two based on the demonic presence at the portal. As early as two months for a demon prince, and as long as two years for a regular demon. Luckily, the chaotic nature of demons means that they don't typically plan out a long-term infection.
Phase Two - A Growing Menace
A demon portal opens to a random location in the Abyss. The portal could open in a desolate land, or in the middle of a horde of demons. Regardless, the portal is a threat to the mortal world as the Abyss begins to seep in. The environment becomes even more volatile: the sky could rain embers, winds could shriek and throw animals, or waters could become acid. It depends what region of the Abyss is pouring into the world.
Demons coming through or hanging around the portal are still chaotic (unless some person/thing organizes them). They fight for dominance over the area. If the presence of demons subside, the portal becomes weaker. If the portal reaches half the amount of demons from its initial formation, it vanishes. Example: 10 demons were present at the creation of the portal. 5 demons are killed. The portal vanishes (This is the official ruling, but to me it makes more sense to have demonic power be a factor here). The infected site returns to normal in some period of time; anywhere from several months to several centuries.
Phase Three - A Stain on Reality
The portal cannot be destroyed. The size of the region grows larger (but not endlessly). Demons might venture away and establish another infection elsewhere. Even if all the demons venture away, or all the demons are slain, the terrain remains infected. But the portal becomes dormant in the lack of a demonic presence at the site of the infection. To keep the portal closed, defenders must establish a permanent presence at the portal. Cultists or random planar energy could reopen the portal at any time.
Phase Four - Apocalypse Now
Demon lord(s) invade. Their power enables them to command the other demons already present in the world. They form an army and begin to strip the world of life. The only hope for survival is the expulsion of the demon lord(s). Demons without a lord will be chaotic again. They'll fight amongst themselves, making it easier for mortals to eradicate them. However, if the demon lord(s) is/are not slain, their incursion will continue until the entire world is taken over and it becomes just another layer of the Abyss.
The Worldwound was the name given to the demon-haunted wasteland at the northernmost reaches of central Avistan between the years 4606 and 4718 AR.[2] It was created when a planar tear to the Abyss opened in the Kellid nation of Sarkoris shortly after the death of the god Aroden, and for more than a century was among the greatest threats to life on the face of Golarion.[3][4]
The Worldwound began its encroachment with strange demonic beasts haunting the barrows and tombs of the Northmounds. These foul creatures soon spread out and attacked nearby isolated clan holdings. While worried, the people of Sarkoris believed that Aroden's imminent return would solve the problem.[5]
Rumours began to spread of a mile-long cosmic blight that was rimmed by jet black flames located southwest of the city of Iz;[5] this site became known as the Worldwound. This corruption soon spread to engulf an area the size of a country, destroying what was once Sarkoris. The Worldwound's unchecked expansion was only halted when the Mendevian Crusades were launched.[7]
The geography of the Worldwound was a sickening, flexible thing, with the features of the land shifting in front of the viewer's eyes. This fluidity of form grew worse the closer one got to the actual rift at the center of this region. The thin veil between Golarion and the Great Beyond tore completely there, and demonic monstrosities poured forth unabated from the depths of the Abyss. At the distal reaches of the Worldwound, where it bordered on the surrounding realms, this instability subsides.[12]
