The Order of the Frozen Skull is a Rogue archetype with optional corresponding lore. It is an order of Assassins that uses strange semi-necromantic cold-based magic and worship death. Players can find themselves having divided loyalties between the adventuring party and this death cult. Or perhaps they stole the secrets of the Frozen Skull from the cult and now they have been marked for death. You might be able to come up with a backstory of your own for how your character got involved with the cult, or you can just use the archetype as-is without any fluff.
Frozen Skull Assassin
Assassins come from many walks of life, and their motivation to kill is usually money, power, pleasure, or a combination of the above. The Frozen Skull Assassins, however, are motivated by their worship of Cold and Death. Their numbers are small but their reach is vast since they seek to spread the “art” of death to as many as possible. They view the assassination contract as holy and work with religious fervor towards fulfilling that contract. You may have many different reasons why you joined this Cult, but there are very few who have managed to leave the Cult alive. Be warned, even if you manage to get away after stealing the secrets of the Frozen Skull, you will be marked for death by all the other members of the organization.
When you choose this archetype at 3rd level, you can use a bonus action to create up to four daggers or up to two short swords made of magical ice. These count as magical weapons. The combat round after you drop or throw one of these weapons, they become ordinary pieces of ice and thus can melt away. Assassins of the Frozen Skull use these ice daggers to practice their deadly art, and then meld into the crowd, leaving the murder weapon behind to vanish in the elements.
When you choose this archetype at 3rd level, when you hit a creature with a Frost Weapon, your target has its speed reduced by 10 feet until the end of your next turn.
Starting at 9th level, your training with Frost Weapons is complete; when you hit a creature with a Frost Weapon, the target can’t regain hit points until the start of your next turn.
Chill of the Grave
When you reach 13th level, your long exposure to the magic of cold and death has left a lingering mark upon you. You become cool to the touch and gain what some would say is an unhealthy pallor to your skin. Your aging slows, aging only 1 year for every 10 that pass. Furthermore, you gain Resistance to Cold and Necrotic damage.
Mark of the Frozen Skull
At 17th level, you learn the final secret of the order: how to mark an assassination target with the Frozen Skull. You choose a creature you can see, and mark it as your quarry for 1 hour. When you do so, a blue glowing skull appears over the creatures heart. While the mark lasts, you deal an extra 2d6 Cold, and 2d6 Necrotic damage whenever you hit with a weapon attack, and you gain advantage on any Wisdom (Perception) check to track your quarry should your prey escape. You must complete a short or long rest before you can use this class feature again.
How to Use the Order of the Frozen Skull
In the ancient days, inhuman spell casters studied and experimented in esoteric magic, in ways that have never been replicated since. These ancient weavers of spells made many discoveries, but much of this knowledge has been lost to the passage of time. The link between the powers of Necromantic magic and Cold magic is but one of the few that has endured to the modern age.
Over a thousand years ago, a depository of this esoteric lore was found by the ancient Wizard Jorun Draven, also known as the Ice Witch. She spent centuries translating the words of the ancient inhuman spell casters, trying to gain as much information as she could. Unfortunately, Jorun decided to use the secrets she had gained to create an army of cold undead. This attracted the attention of the world’s heroes and she was ultimately defeated. These adventurers put all the knowledge Jorun had collected to the torch, save for one book that the party Rogue hid away. The Rogue, whose name has been lost to time, attempted to translate this Arcane lore. What came out of this is the semi-religious writing known as the Tome of Frozen Darkness.
This book had a strange power and called to it like-minded individuals who began to worship Cold and Death. From this cult grew an order of assassins that incorporated the cold into their practice of the deadly arts. In the modern age, the Order of the Frozen Skull works in secret, though they can be a source of terrifying opponents for any group of adventurers. They say some of their members are especially deadly, for they have transcended from their mortal bodies into the ultimate being of Cold and Death — the undead.