The 'canon' story behind Wee Jas' death domain is that when the Suel (the human subrace whose pantheon she belongs to) Empire was destroyed a millenia ago by the Rain of Colorless fire, the dying the dying people cried out to Wee Jas, the goddess of magic and law to guide them to salvation. The sudden increase in her power granted by all these new worshippers manifested itself as a new domain for her.
However, there are two HUGE flaws in this story:
1.> The empire' ruin came about as the result of their own mages doing insane arcane battle with their enemy, the Baklunish people. Why then would the common folk cry out for the goddess of magic to save them, when it is her gift that brought about the trouble to begin with?
2.> Deities & Demigods (3e) EXPLICITLY says the D&D pantheon (read: greyhawk gods) do
not derive their power from the number of people who worship them, though they may tap their worship base for certain activities. How then does the sudden gain of new worshippers grant a deity more power?
Myself and a few others in greyhawk fandom have theorized on the true story of Wee Jas and the death domain. The stories vary, but the usual theme involves her usurping the domain from another, forgotten goddess.
Some details on my account can be found here:
http://www.canonfire.com/html/modules.php?name=News&file=article&sid=105
Another account, this one by Keoland Triad guy Sam Weiss, is found here:
http://www.canonfire.com/html/modules.php?name=News&file=article&sid=156
Sam's is interesting because it invokes the classic 'maiden, mother, crone' triumvirate idea that is common to a lot of mythology.
Note that neither my account or Sam's is 'canon', but they might help you craft your own version of the story of Wee Jas.
By the way though, the canon term for a cleric who follows wee jas is 'jasidan', as in 'I am of the Jasidan faith' or 'I am a Jasidan cleric'