.... grrrr !! I really HATE these guild quests !
My priest has completed the "spider egg" quest, but now my wizards wants to become a warlock. But i cannot find a second spider egg ... ??
Any ideas ?

Ok. Where did you found that first egg?
I met one huge spider at the forest near first town.
Then I met another spider in the windmill near first town too, where Kerielle
or something like that hangs around.
You can find more eggs at serpent temple after you get pass oracles.
Try to find that spider out of the forest, don't know specific location but I think it's near shore.

Spiders start turning up as "wandering monsters" after a while. I had so many spider eggs that I kept throwing them away...

To make sure you get a spider egg next time you encounter a spider, do the following. First, make sure you are turn based. Have everybody besides your theif intentionally miss. I did this by backing up so they were out of range. When it is your thiefs turn, have him pickpocket instead. Eventually you will get an egg, then kill the spider. If you haven't killed the one under the windmill, it is always there. There is also one somewhere by the Gypsy house.