I find it very frustrating (but I find this entire game frustrating) that black wax idol is required for so many thing yet i have such hard time trying get it!
Seriously, i don't understand!
I think it would be helpful if Eris give us some when you do her bounties after you've maxed out your rank with her...
Just a thought.
Edit: P.S. I know of all the ways you can get them but they never drop for me!
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I have around 34 right now. All you have to do is go to Patrol cosmodrome, head the the underground area with 2 hallowed acolytes and 1 knight and kill them. Run upstairs, wait a second and repeat.