Frigid Blade

For many rogues, the chill damp of a rainy night is the worst part of their chosen career, whether they be thief, assassin or spy. For the Frigid Stalkers, though, it is their lifeblood. Whatever their goal, Frigid Stalkers can function in extreme environments that would send a normal person's teeth chattering. They embrace the cold wet darkness and it embraces them in kind. Water refuses to splash as they pass, the chill keeps potential opponents huddled around warm fires. And hidden in the rain, Frigid Stalkers go cooly about their business.

Spellcasting

When you reach 3rd level, you gain the ability to cast spells. See Spells Rules for the general rules of spellcasting and the Spells Listing from all spell lists.

Frozen Fate

Starting at 3rd level, water becomes your ally. Water and snow and icy surfaces do not count as difficult terrain. The water quiets and obscures your passage, you have advantage on Dexterity (Stealth) checks in areas with drenched floors, heavy precipitation, snow, ice or when swimming underwater. 

You can see clearly in heavy snow, heavy rain, and heavy fog.

Additionally you gain a swim speed equal to your walking speed in water at least thigh deep.

Frost Mark

Starting at 9th level, your Sneak Attack damage is dealt as cold damage.

The chill from your Sneak Attack persists, marking your target so they cannot hide from you. When you hit a creature with your Sneak Attack, a splinter of ice forms within them. You can choose to concentrate on the splinter, creating a Frost Mark which lasts for 10 minutes. You know where the creature is within a margin of error of 10 feet at all times. You can focus as a bonus action to locate it exactly, the creature glowing an icy blue to your eyes regardless of any skills or spells it uses to conceal itself. Should you lose concentration or if the creature is ever more than 150 feet from you, the Mark melts and you must use Sneak Attack to create another one.

Additionally, your target takes an additional 2d6 whenever it is attacked while under the effect of the mark.

Cold Blooded

At 13th level, the cold has seeped into your very core. Your skin is cool to the touch, often at or below the ambient temperature. You no longer need to make saves to endure extreme cold. Additionally, you can breathe underwater.

Creeping Cold

Starting at 17th level, your Frost Mark grows with each Sneak Attack, becoming deadlier and deadlier. With each additional Sneak Attack on a Marked creature the Frost Mark grows a level up to a maximum of 5, granting you the following on further attacks against the Marked creature:

Level	Bonus Effect
1	Advantage on attack rolls
2	Weapon attacks score a critical on a roll of 19 or 20
3	Sneak Attack deals an additional 5d6 damage
4	Weapon attacks score a critical on a roll of 18-20
5	Sneak Attack deals an additional 10d6 damage