Parasite

You have specialised on using others weakness to your advantage. 

Vampirism

You gain the ability to feed on other creatures life force. This can be flavored as a blood transference, a magical energy drain or even just a classic vampire bite. Whenever you deal sneak attack damage against a creature you may use your bonus action to transfer any sneak attack damage dealt into hit points, with any excess of hit points just lost. You can use this feature an amount of times equal to your proficiency bonus and it is restored on a long rest, but it can only be used once per combat.

Symbiosis

Your transference of life can also be used in a beneficial way to remove a disease or poison. You can choose a creature within reach, if they are unwilling you must make a melee attack roll, you bring any poisons or diseases from them to you, making the same save to see if it effects you. You can only do this once per short rest. Once you reach level 8, you can do this twice per short rest.

Undetectable

You have gained not only control over hiding yourself from sight, but also from smell, hearing and all other senses. Creatures with either Keen Senses, Tremor-sense or Blindsight do not gain any edge on finding you, but are instead treated as any other normal sight dependent creature would. This means if they are unaware of you, you would have to roll Dexterity (Stealth) higher than their Passive Perception, or if they are looking for you, it is your Dexterity (Stealth) against their Wisdom (Perception).

Improved Vampirism

Your vampirism evolves to be more effective and more lethal. You can now use it twice per combat. You also can choose one of the following effects to happen whenever you do vampirism to a creature. The save DC for all these are effects are 12 + your Proficiency Bonus.

Blinding Leech

You steal the ability to see from the target temporarily. Upon vampirism that creature must make a successful Constitution saving throw, or be blinded, at the start of every one of their turns they may roll the save again to try remove this effect. 

Nightmare Drain

Upon vampirism you release a creatures darkest fear. That creature must successfully complete a Charisma saving throw or is Frightened by you till the end of your next turn.

Poisonous Injection

During vampirism you also release poisons into the target. That creature must succeed a Constitution saving throw or be poisoned, they can make a saving throw on the start of each of their turns in an attempt to remove the effect.

Endless Hunger

You thirst for life force has now grown to massive proportions. Now you can choose to do vampirism, and it will count as a sneak attack.