The Wyrm of Worlds is an ancient and powerful demon lord from the Netherworld. He was originally summoned to the Prime Material Plane by Darnashta'i, a Tsol'aa who hoped to use the demon's power to protect his forest against the advance of the newly created humans. Pretending to be under Darnashta'i's command, the Wyrm used an evil ritual to slay all of the humans in the forest. Next, however, it used the same magic to slay all of the Tsol'aa. Darnashta'i had never had enough power to actually control this being -- it had simply been playing with him. To bring further fear, guilt, and horror to the Tsol'aa, the Wyrm then ripped the forest from its previous location and placed it deep underground, creating a magical portal to provide it with the necessary sunlight.
Before the demon could progress further, however, Gaia manifested and rebuked him for interfering in Her realm, matching wills with him briefly before imprisoning him in a stone deep within the forest of Azdun. As punishment for his sins, Darnashta'i was set as the eternal guard on the bound demon.
The Wyrm was stronger than the earthmother had guessed, however, and he managed to exert some control over Her even from his prison. In the year 209 AF the Wyrm caused Gaia to slay the elder god Twilight. As punishment for this crime, the other gods bound Her within the Gaian Tree, and Twilight, with the support of Eris, brought darkness to the forests in vengeance. It was not until Aurora discovered the origin of Gaia's madness that She, Oneiros, and Hermes assisted mortals in freeing the earthmother from the tree. The energies released from that event opened the way to lower Azdun and the lair of Zsarachnor.
With Gaia's passing, however, the spell that held the Wyrm was removed. Acting discreetly, he quickly bound terrified Darnashta'i into the stone where he had spent so many centuries, and began to move against the emerging power of Demeter. Lacking a corporeal form of his own, the Wyrm choose to invade the mind of Demeter much as he had done to Gaia so many years before. Over time he solidified his control, planting seeds of arrogance and malevolence within the goddess until the day arrived when he finally grew powerful enough to seize control entirely, overthrowing Her in a fierce battle of wills. Demeter was banished to the Netherworld, and the Wyrm retreated into Azdun to recover from the struggle, diverting energy from Demeter's shrines to himself.
In the year 402 AF, the armies of Matsuhama and Mhaldor moved against the neglected shrines of Demeter, causing the Wyrm to once again awaken. Carefully constructing visions, he urged the remaining members of Demeter's Order to rebuild the shrines, and eventually went so far as to manifest in the form of the goddess himself, alarming the forestal community with his bile and venom. Soon thereafter Darnashta'i was discovered in his prison, and the remaining shrines of Demeter were decimated to such an extent that the Wyrm was forced to retreat again into the forest.
In the Netherworld Demeter experienced many horrors, changing fundamentally in Her essence. Eventually She was able to recover enough power to send visions to some forestals, who quickly gathered together to perform a ritual, summoning Her back to the Prime Material Plane. Demeter, in the form of a shrouded woman, immediately challenged the Wyrm. After a brief but dramatic contest, She was able to defeat and bind the demon lord once again, asserting Herself as the reborn goddess Melantha.