Character Creation:
Deities.
The party will be from Greyhawk. So use the Greyhawk gods if your character is of a Divine class.
Stelian:
Stelian is a cleric of Pharasma with a particular dislike for Undead things.
Um is this right?
If you just want a deity who HATES undead, then Pelor is the big name in Greyhawk, the Flan Deity of the Sun. "Pelor is wrathful against the forces of evil, and is especially opposed to the undead. However, Pelor urges his followers to remember that excessive attention to things of evil can blind one to the truly important things: compassion and goodness. These are what must be emphasized above all."
If you want a death deity who doesn't like undead, there's not much Chronepsis fits the bill but he's mostly a Dragon deity, not usually worshiped by humans.
Full disclosure Friedhelm though not overly religious admires Mayaheine the most. Mayaheine was a paladin of Pelor who ascended to demigoddess. But as I mentioned Pelor is possibly the most popular deity in all the Flanaess so he comes up often.