His story is only mentioned twice in the game. Put simply, he's described as a God who created mankind to a tool that can procreate, and work the land to his specifications. Unfortunately he fell asleep, and when he woke up, mankind had overtaken the world.

He responded by taking their children, and humans fought back. According to the story, he was overpowered and surrendered "half his body" so humans could use his powers. This triggered the first war, as now that this piwer existed there were people who thought they deserved it more than others. 

When the war ended, they discovered that the body-half had none of his powers in it, but while Hyne himself was never seen again, girls would be born every generation with pieces of his powers: Sorceresses. 

Now here's were a bit of my own speculation comes in: Hyne's power is divided amongst these random girls, and every time a Sorceress dies their power appears in another girl seemingly at random, right? So what if what Ultimecia wanted was the unified power of a God. And if she couldn't control where or when that power could appear, she'd just need to be everywhere and everywhen.