A Soul Stone is a stone of some crystalline properties that is found in the hearts of the Bazrrod demon lords. Indeed, it might be said that the soul stone is the true form of the bazrrods, their bodies are only their clothing.
Each Soul Stone is a stone that can be held in the hand that has some crystalline properties. Their structure and substance might be unique to themselves as subsidiary demons created from the Soul Shards struck from a soul stone are dependent upon the central soul stone. If the soul stone should be destroyed, the loss would eventually doom the demons created from the soul shards of that particular, unfortunate soul stone.
Speculation: Information might exist within the crystalline matrix of the Stone? Energy acting from a distance might still connect the pieces of the stone?
However, the soul stones would appear to be nearly indestructible, as the humans and elves who defeated the Demon Lords in the First War of the Power didn't destroy the soul stones that they captured, but rather, 'imprisoned' them in Demonatch.
The Bazrrods can create subsidiary demons by splitting off a tiny fragment of their stone and emplacing it in a body derived by magic or another source. While many Bazrrods have created veritable armies of subsidiary demon soldiers, doing so weakens the demon lord greatly.
Demon lords come in different ranges of power as well, divided into Primary, Secondary, and Tertiary categories based on their power (Albeit, this is a socio-political construct as well, so power isn't the only factor). It might be that each soul stone has its own individual size and characteristics which determines the power of the Bazrrod or the number of Soul Shards that can be split off from the central core?
The origins and history of the soul stones is the same as the Bazrrods, and cloaked in mystery. The Bazrrods emerged from the underground, but the true origins of their Soul Stones is a mystery.
Speculation: One theory is that the soul stones are the product of an advanced technology. Perhaps by the 'World Builder' beings who seem to have terra-formed Aiers and many other worlds throughout the Nebulosa of Ots? Or perhaps from another race?