Symbiotic Host

You have been infected by a  symbiotic lifeforms that cannot survive without you.  As it grows it feeds on your life force in exchange altering  your physical abilities to enhance your chances of survival.

It Lives

3rd level 
Your body becomes home to a strange life form which forms symbiotic relation with you aiding you in exchange for nourishment. As the symbiotic passenger grows it feeds on your life force, as a part of long or short rest you may convert a number of Hit Dice into Energy Dice which you can use to fuel Symbiotic Host features. After a long rest, you must convert enough Hit Dice to bring the number of your Energy Dice to half your fighter level. If you fail to fully feed the life form this way within 24h you gain a level of exhaustion.

Pain Killer

3rd level
Symbiotic life form manipulates your perception of pain. Once per turn When you take damage, you can use your reaction to expend one Energy Die, roll the die, and reduce the damage taken by the number rolled plus your Constitution modifier (minimum of 1).

It Hungers (Optional)

3rd level
Once per long rest you can use an action to extract nutrient from a creature within 15ft of you. The target must make a DEX saving throw (DC equals 8+your PB+DEX or STR) or become grappled by you and you both become restrained. The affected creature or any other creature can make an Athletics check (same DC) to break this grapple. While the target is grappled in this way the life form feeds on it extracting up to one Energy Die per 4 of your fighter levels (rounded down) and the victim takes necrotic damage equal to the roll of the extracted Energy Dice. Total number of Energy Dice that can be extracted from a single creature depends on its size.

Tiny -------> 1
Small -------> 2
Medium ---> 4
Large -------> 6
Huge -------> 9

You can use this action only if the number of remaining Energy Dice at your disposal is fewer than your PB. While you have no Energy Dice remaining the life form may attempt to urge you to take this action upon nearest viable target, friend or foe. You can resist the urge with a successful CON saving throw against DC equal to 10+number of hours since the last Energy Die was expended. 

Survival of the Fittest

7th level
Adapt
As an action you may expend an Energy Die to gain benefits of Alter Self spell for 10 minutes (No concentration required). You can use this feature number of times equal to your Constitution modifier. You regain all uses upon completion of a long rest.

Limit-breaker
The symbiotic passenger temporarily enhances your biological process overriding your mental blocks and allowing you to exert your body to its limits. Once per round you can expend an Energy Die immediately after you made an Attack roll, saving throw or ability check using DEX, STR or CON. If you do you may roll the die and to add it to d20 roll. If the attack still misses, you fail the ability check, or the saving throw you take damage equal to the number rolled on the die.

United in Flesh

10th level
Once per long rest, as a bonus action you can expend a number of hit dice equal to or fewer than half your PB to activate United in Flesh for 1min. While this feature is active you must expend an Energy Die at the beginning of each of your turns to maintain it

While United in Flesh is active you gain the following benefits.

• Each turn, for every Energy Die spent to activate this feature  you have advantage on a single attack made as part of the Attack action.

• Once per turn when you hit a target with a weapon attack, you can add extra d10 force damage per Energy Die spent to activate this feature.

• You can add the number of Energy Die spent to activate this feature  to any DEX, STR and CON ability check or saving throw.

Will to Live

15th level
When your hit points are reduced to 0 you may expend any number of Energy Dice. If you do you are immediately stabilized and the symbiotic life forms takes the control of your unconscious body. It spends every turn trying get you to safety and absorbs all damage up to the number rolled on the Energy Dice spent to activate this feature. It can only take disengage, dash or dodge action and it spends all movement trying to get away from danger. At the end of each turn you roll a d20 on if you rolled 10 or higher for the third time since falling unconscious you regain 1 hp. When you regain consciousness you also regain control of your body. Once this feature is triggered it cannot be used again until you'd completed a long rest.

 

Do or Die

18th level
Once per long rest, when you would drop to 0 hit points you can roll all unspent Hit Dice and your hp becomes equal to the number rolled instead. You gain number of Energy Die equal to number of Hit Dice expended this way. Lastly, United in Flesh becomes activated (even if you used the feature already or if the life form is dormant) as if you spent 3 Energy Dice to do so.