"As an individual, defeat Talen-Lah without losing the Bone Rot effect when Talen-Lah and Bar-Sakka fight together at the end of the fight in Veteran Naj-Caldeesh."

As noted in Incremental Patch 1 for u47, this was patched to prevent completion on Normal. But since the patch, I think it's no longer possible to complete on any difficulty.

Our group in vet NC made sure to all have the Bone Rot 12-second DoT effect going into execute. In this fight, Talen-Lah returned at 5:53. You can see that we all retained the effect until the end of the fight:
https://www.esologs.com/reports/WyP1FzQ736dAZvJN?fight=last&type=auras&spells=debuffs&ability=242251&start=352598&end=364903

This achievement is part of the Naj-Caldeesh Champion meta-achievement. Please fix it or let us know if we're doing something wrong!