Isle of Lost Magic
The Plane of Convergence was once a nameless demiplane in the Astral Sea which served as the dumping grounds for many of the gods’ failed creation attempts. Mechanicals, living creatures, undead, all types of creatures that were deemed failures were abandoned here.
The Banishment of Rubriman
Rubriman was banished to this demiplane centuries ago. At the time, he was a divine servant of Gond, with large amounts of knowledge and magical power at his disposal. After his banishment, his magic was stripped from him due to Gond rescinding his favor of the man due to his experimentation.
Rubriman had, however, studied many things in his long career as a priest of Gond. Planar nature was something he had studied extensively, hoping to bring the worship and craft of the Gearsmith to many different races. Rubriman had been banished to a plane that, while occupied, was not finished forming. A supreme act of will combined with the worship of his followers allowed him to change the plane significantly, forming a true plane at which he was the center. Doing so elevated him to near-deific power, and he began to convert the creatures that inhabited this forgotten plane to his faith and his methods. All life on the plane was subsumed into the mechanical augmentations of the faith, and the plane came to be known as Convergence.
Rubriman enforced his power through the use of artifacts he developed known as servant stones. Once implanted in a creature, whether living or converted, it allowed Rubriman and his commanders to give unquestioned orders to the creature. This also allowed him to strip the idea of other gods out of his converts’ heads, forcing them to worship him as the only god in the multiverse.
For all his power, though, Rubriman failed to find a way to open a portal out of the plane. Because the plane was created as essentially a deific garbage can, the only way in or out was at a god’s will. He spent many long years experimenting with creatures that had inherent magical or psychic powers, trying and failing to develop a creature that could tear a rift in space and allow him and his faith to march out and begin conquering and converting every living creature they could discover.
Eventually, his answer came to him in the form of the Spellplague. When magic died and was reborn, the links among the different planes were shattered and reformed, and links had been formed between Convergence and the Material Plane. These links were weak, having just been formed, but were ever so slowly gaining strength. Soon, he had the ability to send single creatures through, then small raiding parties. Not long after, a commander known as Karthan found a way to reinforce the link from the Material Plane’s side, creating a stable rift that could allow many creatures to come through.
With this method now firmly in place, Rubriman began assembling his armies in Convergence, waiting for the day that his faith would spread across the Material Plane and from there, the multiverse, converting all.