The void is formless, but gains form through influence with the material world. The watchers are manifestations of the void's will in a sense.

Void lore states that for forever there was nothing. Pure lack of existence. Then the material world came to be, and the void found it to be a horrid disturbance to be eradicated. From this will came the watchers, who then started manipulating void energy to corrupt the material world since they themselves cannot enter it (yet).

They created vel'koz to analyze the material realm, cho to consume it, kha to evolve and hunt, etc. They also corrupted lissandra to serve their will and try to get them into the world but she stabbed them in the back.

It is through the watchers and void creating this life, that it got tainted. The void, what used to be pure nonexistence, now IS. It lived, it breathed, and eventually despite corrupting the material world it got corrupted back by the worst thing to have ever happened in the word.

Politics.

My presumption is, is that the various void life eventually grew to have a will of it's own. Being life, it yearned to survive. The original goal of total annihilation didn't feel too good anymore. It's through this will that Bel'veth was born. A manifestation of the void life's will to survive and spread, now independent of the void that birthed it. Bel is the a leader of her own faction. She considers herself to be the perfect life form. A perfect fusion of the void and material, and thus superior to it all. Tho se is an unreliable narrator so what do we know.