I don't know if this is the right board to post this in but since it's Rules-related...
I was wondering if I could create a campaign based on the child of Bhaal story. Actually, I wasn't thinking of turning one of my characters into a Bhaal spawn. Maybe, however, he could be suspecting that and should go for a 'test'? Or he could help an NPC find his true heritage? Would this work OK in P&P or would it be way too complicated?

Again if it's the wrong forum let me know.

No, it's the right forum. I don't see why you couldn't create a campaign based on that. You have a wealth of information at your fingertips (just play the game ;) ), so it shouldn't be too much of a problem. Still, if this is your first attempt I'd rather start with a normal party without any special characters such as Bhaalspawn, Chosen of Mystra or any such sort of characters that usually need the story to revolve mostly just around them.

Also, there are no handy "Bhallspawn tests" to tell you if your character actually is one. One must usually find that out for himself.

Thanks for the advice Taluntain but I have come to realise that since most of my friends have played BG and BG2 several times this wouldn't be such a fascinating adventure :(. Besides, I don't have the time now. If anyone would find it interesting let me know. I may get around it in summer. Btw, what do you think would suit it better 2E or 3E rules?

It wouldn't really make much difference. If you're only familiar with 2nd edition, rather use that. It might be a bit too much to start that kind of adventure in rules you're not familiar with. Or not... It's really totally up to you.