Sundering of the Heavens

The Sundering of the Heavens is the name given to Vanth's splitting of the Ethereal Plane into Elysium and the Abyss.

The First Archfiend
Vanth felt stuck at the base of the Ethereal home of the gods, the mountain of Elysium. She was bound to accept beings from the Material Plane as they died, guiding them from the Plains around Elysium to the Wall on which they'd spend eternity in rest. For thousands of years, she felt jilted and isolated, becoming more and more jealous of her brothers and sisters and their powerful creations. That all changed with the death of a powerful elven necromancer named Vethdon.

When Vanth greeted Vethdon on his arrival on the Plains, he didn't want to go to the Wall. He stalled and slowed their progress to the Wall with stories and questions about her power. Eventually, he offered her a bargain. He could teach her to create beings of her own out of the souls of those she received if he could be the first. Vanth was intrigued. They spent years working on a ritual, testing it on newly arrived souls to the Plains, many of them being destroyed and banished to the Void in the process. Eventually, they perfected it and were ready to try it on Vethdon. The ritual was successful and on his ascension to semi-godhood, he swore fealty to Vanth who deemed him her first Archfiend and gave him a new name, Asmodeus.

Point-Counterpoint
As Vanth perfected the ritual, she and Asmodeus created more and more Archfiends and found a way to create lesser demons out of souls lacking the power to be Archfiends. Soon, she was able to use them to usher souls from the Plains to the Wall and spend more of her time building her army and improving her own power. The other gods soon noticed what she was doing, but were unsure if or how to stop her. Aworan finally spun a prophecy that depicted Vanth and her Archfiends ravaging the Ethereal Plane and taking control of the Material Plane. Draach was the first to act, finding a way to elevate one of his dragons to what he deemed a Celestial. He spread the knowledge of this ascension to the other gods, who quickly built up a small collected army of their own Celestials to do battle with Vanth's Archfiends.

The Sundering
Vanth and Asmodeus came up with a plan: she found a way to split the Ethereal Plane in two. By sundering the plane, she believed she could trap the other gods and their Celestials on Elysium and at the same time, open a way for her to send her Archfiends and demons into the Material Plane and eventually even herself. She enacted her plan and managed to break a rift in the plane. The Plains and the Wall split from the great mountain and formed their own plane, which she called The Abyss. However, she miscalculated the outcome. She created a gate to the Material Plane, but it created an identical gate on Elysium and the other gods were not trapped. Now both the Celestials and Archfiends could be sent to the Material Plane and maybe even the gods themselves. With her own plane to command, Vanth began to accelerate her efforts of creation, but would bide her time, waiting for the ideal moment to make a move on the Material Plane and crafting a plan to do the impossible: destroy Draach.