I Fog bombed Linder Kemm while he was still in the magister barracks to see if it would change the vault fight. Possible that he specifically cant be hurt by it. All his paladins went down easy enough.
He's an undead using a shapeshifter mask so thats kinda normal he can't be affected by it. Which in turn makes one realise that if you use the shapeshift mask and go undead, you ll never be affected by deathfog either