Underworld/Black Widows

First off i have searched the forums and my answer hasn't been answered so ill post here. I do know that you can get Elder/Dire Black widows in underworld but what i want to know is what is the trick to getting them and does anyone have any suggestions. the reason i ask is that i dont want to screw the group im in because i dont know what im doing. so if one anyone has tips for getting one that would be helpful thanks.

Complete the "Wrathful Spirits" mission (I think that's the name of it) and when you are supposed to go to the Reaper to get your reward, there will be a spider wandering about near the statue of Grenth along with the chest it came out of.