Dark Devotion. The fanatic has advantage on saving throws against being charmed or frightened.

Mist Rot Bomb - A fog of toxic mist that once inside you must make a Con Save 13 - If failed you become paralyzed can remake the save every round. Otherwise, you take 2d4 poison 2d4 necrotic. If you reach 0 HP in the mist you become an undead version of yourself and must attack closest allies or enemies. Greater Lessor Restoration can get ride of the effect. 

When taken to 0 HP a small gadget releases a small beaker and creates part one of the mist rot when combined with the other half the undead mist assassins will come back to 5 HP until commanded 

Multiattack. The fanatic makes two melee attacks.

Dagger. Melee or Ranged Weapon Attack: +4 to hit, reach 5 ft. or range 20/60 ft., one creature. Hit: 4 (1d4 + 2) piercing damage.

Sneak Attack. If the Assassin has thus far gone unnoticed it will always use a sneak attack, when it first strikes, attacking with advantage and gaining 2d6 bonus damage.