The Hero of Wrothgar or at least part of the acheivment prerequisite acheivments are bugged. In my DLC acheiment list for Wrothgar, the Wrothgar Dungeon Slayer achievement is shown as unfinished with Old Orsinium Vanquisher being complete but not Rkindaleft Vanquisher. However, the Rkindaleft Vanquisher shows finished in the same list.



This being said I believe the reason stemmed from a specific issue. While in Rkindaleft, I attacked Wondering Illia. I ran from the battle to reset them. As they returned I shot at Illia just as they momentarily disappeared. They then did not return to their locations. I left and came back hours later and they were there. Upon killing the boss it gave me the achievement. The name was checked off but still greyed out. This morning as you can see it's no longer like that after killing it again.