I can only access Mantol Derith on characters who have completed the Dwarf King questline. My oldest characters did it as a matter of course and my newest characters did it because I refused to pay out for Dwarven Spelunking and never finished Underdark.
In the screenshot, you have completed ME but not Dwarf King. The change in requirement may be a bug but you should be able to workaround by doing the adventure.
"We have always been at war with Dread Vault" ~ Little Brother
I am experiencing this also. Characters that have compled the underdark campaign(this is from before it converted to an adventure) that have not also completed the dwarven king storyline cant access Mantol Derith. It requires you to accecpt a quest from Captain Belgold that he doesnt offer.
This seems to be similar to the bug where you couldnt make greater demonic keys in the underdark campaign with characters that didnt do the dwarven king saga
Tested this morning on a character that had completed Underdark and Maze Engine but not Dwarf King. Mantol Derith was locked. I ran through all nine chapters and got the quest to meet Drizzt, then was able to go to Mantol Derith as previously.
"We have always been at war with Dread Vault" ~ Little Brother
Comments
In the screenshot, you have completed ME but not Dwarf King. The change in requirement may be a bug but you should be able to workaround by doing the adventure.
This seems to be similar to the bug where you couldnt make greater demonic keys in the underdark campaign with characters that didnt do the dwarven king saga