The Sunken Hunter

Wherever breathing becomes a hindrance, these hunters can be found. Deep caves with little air, beneath swampy waters, even in the far reaches of the astral sea- these specialized rangers have learned special techniques to hold their breath for extended periods of time and eventually turn this into an effective tool for all areas of their work.

Held Ambush

Your time rigorously training your body to hold air for long periods have time and learning to move as little as possible to expend as little energy as possible have made you an adept ambusher. Starting at 3rd level, you gain advantage on initiative rolls and advantage on stealth checks provided you are holding your breath and remaining still. Finally, you can hold your breath for a number of hours equal to half your level (rounded down) before you start to suffocate.

Practised Toughness

Time spent learning your body has granted you greater defense against effects that would attempt to harm it as well as the best ways to move through usually difficult places. Starting at 7th level, you gain proficiency in constitution saving throws as well as resistance to poison damage. Finally you gain a swimming speed and climbing equal to your walk speed.

Steady Technique

Your training allows you to make your strikes more precise while employing them. Starting at 11th level, at the start of each of your turns if you are holding your breath you can choose to gain advantage on attack rolls until the end of your next turn. If you do so, you must make a concentration check whenever you take damage while this effect lasts. On a failure you loose this advantage and cannot choose to gain it again until the end of your next turn.

Patient Master

At your apex, your hardiness and training meld together. Starting at 15th level, whenever you fail a constitution saving throw you can use your reaction to succeed instead. You can use this reaction a number of times equal to your proficiency bonus, and regain all expended uses at the end of a long rest. In addition, the first time you hit a creature on your turn that hasn't acted yet in combat you can choose to turn that hit into a critical hit.

Finally, whenever you make a constitution saving throw to take half damage on a success you instead take no damage on a success and half damage on a failure.