Arpenia dungeon (near Leyawiin) that you visit during a fighter guild quest (azani blackheart) has a room with multiple poison vents.
These vents apply a poison to the player and the NPC Modryn Oreyn that is not removable with ANY effect (neither spell nor potion) and lasts until it kills the player.
The NPC Modryn Oreyn will be knocked unconscious over and over and over endlessly.
The only solution i was able to find from other players is to reload to an earlier save before the player or the NPC has not been infected - and then to completely avoid that room (i think its in the bottom left corner of the Arpenia dungeon).
My apologies if i am missing something.
Thank you for all your work on the Bethesda games!