After one character walked a nearly empty dungeon, then used NIHIL on the Arch Cultist and nonchalantly showed their Mandate to Verhu, I decided a few more tweaks were in order to gently ramp up the difficulty. 

To wit, more traps, less empty rooms, and a few other minor adjustments to the rules - Arch Cultist impervious to Nihil (but you could still use Hellghasts, bombs, etc), Tough Monsters are now in the pit trap, and the saving throw for the blinding flash had been removed.

Enjoy!