On Torment 4, after progressing the Kichuk Stronghold up until the last phase which activates the Dweller in the Dark, the various paths become blocked off and the intention is to damage the Dweller in the Dark several times as the paths open up and the player progresses through the north of the stronghold.
However, after damaging the Dweller until their health was low enough to initiate a phase change, they became invulnerable and simply continued attacking me. The stronghold event was incompletable.
The cause was likely putting the Dweller into the staggered state as their health passed the threshhold which should have triggered a phase change and caused the NPC to flee, opening up another path to progress through the stronghold.
This bug would have ended in a hardcore player being killed due to no fault of their own. Please rectify this bug immediately. For hardcore players, please avoid this stronghold.