Ratamiel

It's the island in the south-east corner. The island is shrouded in fog and occupied by fampyrs and undead.

I'm having the exact same issue, we even posted about it at roughly the same time. https://forums.obsidian.net/topic/99954-a-paradise-of-the-mind-bug/

Description: After I killed Menzzago and destroyed the orb I can't complete the quest (and obviously can't get the achievement as well) by speaking to Copperhead. He just says "Good luck, eh?" when I press on him, but I don't enter into actual dialogue. Also I have both the "Ascend the stairs within the crypt." and "Speak with Copperhead" parts of the quest open in the questlog. I've included a screenshot. Steps to Reproduce the Issue: I came to the island and cleared the areas right up to the one before the Orb area. Then I stopped because I wanted a couple more levels and came back later. I'm unsure if this may be why the quest is bugging and won't let me complete it now. Important Files: I've included a save just before I ascend to the orb area and kills Menzzago and a save where I'm standing at Copperhead after killing Menzzago and destroying the orb. I also included a screenshot of the questlog showing two parts of the quest uncompleted at the same time. https://www.dropbox.com/s/lb5e0mqdxhpyq96/Pillars2bug.zip?dl=0 If you have any questions or need more info then just ask!