Read ther story of Hyne here:

http://www.geocities.com/aarinfantasy4/guideint3a.jpg

To summarise:

Hyne was the first sorceress. The term implies female gender, but he is called "he" as well, so it's impossible to tell. Hyne may not have had a specific gender at all. Anyway, Hyne seems to be some sort of demi-God. He created human beings to function as his slaves, yet he was not immortal, and could be killed by humans. He did, however, almost certainly have infinite lifespan. That's why he could take a nap, and wake up to find the human race to have grown to an uncontrollable level. To fix this, Hyne starts killing children, which naturally pisses the human race off.

Sometime after their fight, or in the midst of the fight, whenever, Hyne distributes his power(uncertain whether he gave away all his power or just a portion of it) to several women/girls, who become the first sorceresses.

And that's all there is to it. Sorceresses all have a fragment of Hynes power, simple as that.