If you like the idea of the Order of the Frozen Skull but find they don’t have a place for your players, here is a monster you can use instead. This is basically an alteration of the Assassin found in the Monster Manual but with some of the abilities of the archetype, removing the poison and adding cold damage. If you want to make more monsters with this fluff, like a high ranking priest or archmage of the Order, just replace some of their abilities with more cold or necrotic damage spells, and give them Resistance to Necrotic and Cold damage.
Frozen Skull Assassin
Medium humanoid (any race), any non-good alignment
Armor Class 15 (studded leather)
Hit Points 78 (12d8 +24)
Str 11 (+0), Dex 16 (+3), Con 14 (+2), Int 13 (+1), Wis 11 (+0), Cha 10 (+0)
Saving Throws Dex +7, Int +5
Skills Acrobatics +7, Deception +4, Perception +4, Stealth +11
Damage Resistances Necrotic, Cold
Sense passive Perception 14
Languages Thieves’ cant plus any two languages
Challenge 8 (3,900)
Frost Assassination. Once per turn the Frozen Skull Assassin deals an extra 14 (4d6) damage when it hits a target with a weapon attack and has advantage, or if they attack with Frost Weapons.
Frost Strike. The Frozen Skull Assassin reduces the speed of any creature it hits by 10 feet until the end of it’s next turn. Also the target cannot regain hit points until end of its next turn.
Frost Weapon. As a bonus action the Frozen Skull Assassin can create daggers or shortswords made from ice to use as weapons. These Frost Weapons are considered magical.
Nimble Escape. The Frozen Skull Assassin can take the Disengage or Hide action as a bonus action on each of its turns.
Multitrack. The Frozen Skull Assassin makes two shortsword or two dagger attacks.
Shortsword. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 6 (1d6+3) piercing damage.
Dagger. Melee or Ranged Weapon Attack: +7 to hit, reach 5ft. Or range 20/60 ft., one target. Hit: 5 (1d4+3) piercing damage.