You've sworn your services to a powerful Unseelie Fey or the Queen of Unseelie Court, unlike a Warlock who gets Arcane Power from such a covenant, you gain different powers.

It's said only a fool enters into bargains with Fiends or Fey. You must be one, you must be desperate and/or your master must have some specific purpose for you. Regardless, you've made your deal and there's no way out now. Welcome to the Gloaming Court, some call it the Winter Court, where to weak die and the strong get stronger.

Sworn to Winter

You've made your deal with the Unseelie Court and begin gaining power. The first rule of Winter is to survive, as such you learn to strengthen your defense with the essence of Winter.

When you choose this Archetype at 3rd level you can call upon winter strengthen your defense. You gain the Mantle of Winter ability.

As a Bonus Action on your Turn you may encase your armor with frost and ice crystals. While encased in the power of Winter you gain temporary hit points equal to twice your fighter level . This effect lasts for 1 minutes in cold environments, half as long in hot or temperate environments. You can use this ability three times at third level and gain one additional use at level 7 and another at level 15 for a total of 5. These temporary hit points increase at the start of your turn by an amount equal to your Wisdom Modifier and may not exceed the maximum number of temporary hit points that the Mantle of Winter provides.

You may make at Constitution check at a DC 10 to extend the duration longer, it doubles the time you can maintain a single use of Mantle of Winter. The DC increases by 5 for every time you make the Check. Once you fail a check or turn off Mantle of Winter after extending it you suffer one level of exhaustion for each time you extended it.

Bonus Proficiency

When you choose this archetype at 3rd level you gain the following benefits.

Your deal with the Fey grants you the ability to communicate in their native tongue. You learn to read/write and speak Sylvan language.

You also gain proficiency with Wisdom saving throws. Winter isn't a place where the weak of will can survive. 

Warrior of Winter

Your bond with Winter grows stronger and you gain more power. You begin to learn how to harness Winter's might to bolster attacks as well as gain some resistance to the harness of Cold environments and damage. 

At 7th Level you when you call upon winter it also strengthens your offense. 

While your Mantle of Winter ability it active your weapons are also covered with Winter's Power. You also deal bonus Cold damage equal to 1/2 (rounded up) your fighter level as frost and ice encase your weapons. This effect will be applied you your unarmed attacks if not weapon is used and causes the unarmed attacks do 1d4 + Strength Modifier Cold Damage as icy claws grow from your fingers.

You gain resistance to Cold Damage and can survive in temperatures up to  -75 degrees F (-60 C) without protective gear, doubled with protective gear.

Further more you can move freely across icy and snowy terrain without penalty.

Veteran of Winter

Your powers continue to expand and grow and you may now extend your protection from Winter's Ill effects to those you choose to protect. 

While calling upon this new ability a light snow seems to fall around you, though it quickly disappears upon contacting the ground, the air around you seems to almost to freeze the skin and your breath leaves a mist in the air.

At 10th level you gain the Cold Front ability.

You gain the ability to activate an Aura that extends your protection from Winter's Ill effects to creatures of your choice. The Aura has a 30 foot. range and gives up to your Wisdom modifier of other characters resistance to Cold Damage and can survive in temperatures up to  -75 degrees F (-60 C) without protective gear, doubled with protective gear. You can at anytime during your turn on or off the Aura or choose which creatures are being affected by it (no action required).

You gain Immunity to Cold damage and your Mantle of Winter grants you Resistance to Fire while active and treats Temperate environments as Cold instead.

Seek Council

At 10th level you gain the ability to try to get mystical aid in the form of information from the Unseelie Court. Beware, the being you contact will likely ask for compensation for the answer and will never give a straight answer. 

You gain Seek Council ability.

Once per day you may cast the Divination spell as a Ritual but instead of a God or God's Servant you reach out the Unseelie Court for your answers. The Fey whom answers may demand additional payment is at the DM's discretion. The cost can vary depending on the how obscure the information is and which Fey answers . Regardless of Cost the answer will never be a straight forward answer.

Creature of Winter

As your power grows you are able to manifest more of Winter and the Power of the Unseelie Court. Your power also changes you, you are no longer what you once were and are now a creature of the Fey.

At 15th level you Gain the Fury of Winter Ability - 

While your Mantle of Winter is active the area around you in a 15 foot area is difficult terrain for those you aren't protecting with your Cold Front ability and will even freeze water enough for your safely walk on. As a Bonus action on your turn you may cause one of the two effects below to happen.

• You can create a freezing wind in a 30 foot cone to blow from in front you that causes ranged attacks to have disadvantage if they pass threw the cone and creatures in the cone to make a Constitution saving throw or be pushed back 15 feet and move at half speed till your next turn. The DC this ability is (8+Wisdom Modifier+Proficiency Bonus).
• You may cause a shard of ice to form and launch itself at an enemy within 60 feet. The target and each creature within 5 feet of it must succeed on a Dexterity saving throw or suffer 5d8 cold damage on a failed save and half as much on a successful one. The DC this ability is (8+Wisdom Modifier+Proficiency Bonus).

Your characteristic type changes from Humanoid to Fey and may detect any active portal or pathway to the Wild Fey with in 50 miles. You can't lose your memory from leaving the Wild Fey and have advantage on Wisdom Saving Throws to resist charmed and  frightened saving throws.

Champion of Winter

You've proven your worth and mastered the Power of Winter given to you.

At 18th level when your Mantle of Winter ability is active you gain the follow benefits.

• Your attacks score a critical hit on a roll of 19 or 20 and the target that you critical hit must make a Constitution saving throw or suffer disadvantage on Dexterity and Constitution saving throws until the end of your next turn and can't use reactions as the bite of winter freezes them. The DC this ability is 8+Wisdom Modifier+Proficiency Bonus.
• Anyone caught in your Fury of Winter cone or is hit by one its ice shards make a Wisdom saving throw. On a failed save, a target becomes frightened of you till their next turn. The DC this ability is 8+Wisdom Modifier+Proficiency Bonus.
• Mantle of Winter treats all environments as cold for determining how long it may last. 

If you have no uses of Mantle of Winter left after rolling initiative you gain one use of it.