Its a one time (optional) orgy. After Shakala's father attacks the tribe, most of this guys end up dead.
Obeah questline starts after you have a population of >60 goblins. Get the Ancient's Book the shaddy guy in the Inn offers you to keep track of goblin population (not mandatory, but really handy).
I guess Obeah name appears because her questline will start even if you don't have Second Mates (but it takes longer).
If you have the Decensor patch (incest/bestiality) and Shakala got her Daughter, you will have to complete Shakala's Daughter questline for Obeah to appear.