The Witherbarks worship a spider god, and from her they collect their most potent venom.  So if you want the venom, then you must summon their god!$B$BTo do that, you must know her true name, and that name is on a tablet buried in Tanaris, in the troll city of Zul'Farrak, in a tomb guarded by Theka the Martyr.$B$BDefeat Theka and find and read his tablet to discover the spider god's name.  When you know it, return to me.