In the early 400s CT, the City of Xytherra was falling into decline. In the preceding decades, the Intara river which watered the roots of the huge memory trees the city was built in, and brought steady trade traffic, altered course. The great monsoons of 368 caused massive flooding which ultimately resulted in a new rivers channel developing about 13 miles to the west. This diverted most trade traffic as traveling on foot was difficult in Xytherra forest, and roads were unreliable and labor intensive to maintain in the tropical climate and frequent flooding. Xytherra did remain an important destination for scholars who traveled to study and use the great memory tree archives. 

A drought lasting approximately a decade in the 420s, caused many of the grafted fruit-bearing branches to die and made canopy basket gardening unproductive. The population decreased by as much as 30% over the course of the drought due to emigration. The drought ended in 433 and was followed by 4 years of excessive rainfall, causing massive floods and destruction unlike the accustomed periodic flooding experienced in Xytherra forest. 

In the early 440s, visiting scholars began to record disturbing experiences while consulting the memory trees. The earliest remaining record of this phenomenon was from Burhalm Altan, scholar of Genna in 452. He described having a nightmare-like experience during his studies. He refers to other scholars who had witnessed or experienced similar events for decades, but any earlier records of this have been lost. He also described a mysterious blight affecting the memory trees, causing some trees to lose a portion of their foliage. 

The nightmare-like experiences and memory tree blight are considered the earliest verified indicators of the onset of the abor fungus, also referred to colloquially as madness mold. It is believed by some experts that Xytherrans had been experiencing the effects of the fungus as early as the post-drought years in the 430s. This is corroborated by records of strange encounters with Xytherran residents, both in Xytherra City and around the world. The historical reports of these strange encounters vary greatly with anti-social behaviors (examples are not limited to: unwarranted aggression, sudden fixation on self harm, speaking in tongues, irrational anger directed at others or the self, delirium and confusion) and physical symptoms (examples are not limited to: uncontrollable vomiting, glowing eyes and veins, blotchy greenish-black coloring on the skin, loss of appetite, fever, uncontrollable trembling). Other experts question the validity of this theory given the wide and inconsistent nature of the behaviors and symptoms, and because the effects of abor fungus on the human body are unknown, with the exception of the multiple concurring accounts we have of nightmare-like experiences. 

In 461, the great medical scholar, Markus Platkamin, traveled to Xytherra City to study these strange reports and described afflicted Xytherrans as screaming, thrashing and sleep walking during the night. He discovered that the victims were unable to be woken until daylight and all described terrible nightmares of being trapped in the earth or strangled and choked by roots. He also seemed to indicate a significant increase of early onset dementia in the populous and physical symptoms like discoloring of the skin, fever, blindness, and deafness. He returned in 464 to continue his studies on what he considered to be a “mind sickness” that spread into the body. He was accompanied by 8 of his under-scholars and an unknown number of students. The party was never heard from again. It is widely believed by historians that the party succumbed to the same affliction as the rest of the Xytherrans. The last documented contact with the city of Xytherra was the receipt of a shipment of unknown exotic fruits to a Genna merchant in September of 464, a few weeks to a few months after Platkamin is believed to have reached the city. The shipment was recorded as “spoiled” in the customs logs. 

Expeditions by scholars and allied nations to investigate, contact or render aid to the city frequently did not return. The remaining records of survivors of these expeditions describe encountering a giant grey mats or webs with a “fuzzy seeming texture”, draped over the memory trees like “fabric covering furniture to protect it from dust”. Today this is understood to be a description of a fungal biofilm. Multiple accounts also describe seeing dark, human and animal-like shapes moving within the biofilm.

Historical accounts describe how the fungal biofilm would release clouds of dusty spores when touched. It is recorded that individuals who breathed these spores or touched the biofilm, became feverish and died in agony within 1-4 days. There is one famous account from the deposition of an unnamed expedition survivor of an individual called “Marna” who attempted to cut through the biofilm and enter the former city. He died within minutes and was vividly described as being consumed by the fungus. Historians debate the authenticity of this account. 

Attempts to investigate the remains of Xytherra City ceased around 470. In the following decades, reports of strange sightings of deformed humans or animals covered in black fungus grew increasingly frequent in the hills surrounding Xytherra forest.

In 495, the Verdallion village of Adder, believed to be about 4 miles outside of the Xytherra forest, was found to be covered in fungal biofilm in the same manner as was described of Xytherra. The King of Verdallia, Georgam III, with the support of the parliament proposed setting fire to the Xytherra forest to rid the region of the biological threat. The Ruling Council of Carldid, the other nation bordering the forest, agreed to support this plan. The remaining healthy forest was slashed and burned, but the fungal-infected areas burned quickly. In less than a decade the forest was eliminated and memory trees were declared extinct. 

The Intara River was described as being grey and black with ash and spores for the following decades and the local flooding increased to catastrophic levels. Eventually this led to the construction of the Intara Dam and the creation of Xytherra Lake, completed in 554.