As a Witch Doctor, you transferred the magic you kept in your Wizard's Spell Book to your skin, through carving or ink. Only other Witch Doctors can read this writing without using magic. If magic is used to read the scrawling, that creature will be mentally scarred. You don't need to learn spells, as you know all, but you need to train your body to be able to use these spells. What you gain out of adventuring with a party is the experience and training required to wield some of the more powerful spells carved into your soul.

Forbidden Knowledge

Beginning at 2nd level, you don't need to prepare spells and spells do not require spell slots. The spells you cast are considered always prepared, but to cast them you must perform your Health Siphon. You know all the spells in the Wizard Spell List, but can only cast them at their lowest level. Any "At Higher Levels" effects don't apply. You cannot naturally cast any spell as Ritual, even if it has the Ritual tag. Additionally, your Spell Casting Ability becomes Constitution. You also gain proficiency in Constitution saving throws.

Health Siphon

At 2nd level, when you cast a spell, you must draw life force from a living thing you can touch. It requires a Bonus Action to touch a living thing, but that living thing can be a plant, animal, enemy, ally, or yourself. A non-willing living thing must make a Wisdom saving throw against your Siphon DC (detailed below). On a fail, this living thing takes the full Siphon Damage (detailed below). On a save, you and the living thing each take half damage. If the other living thing has less health than the Siphon Damage, the excess is dealt to you. Unconscious, Frightened, and Charmed creatures have disadvantage on the save. Non-sentient plants automatically fail. Willing creatures don't make the save and take Siphon Damage as if they failed the save, but will not be reduced past 1 hit point. Excess is dealt to you.

Siphon DC: When you draw life force from a creature they have a chance to resist it. The DC for this save is equal to your character level+your Constitution modifier. This DC is different than your Spell Save DC, which is still used when casting the spell.

Siphon Damage: The damage a creature takes when you siphon their life force is equal to the spell's level*10 (a number of d20 equal to the spell's level)+Proficiency Bonus of Necrotic damage. Damage must be dealt to some creature for the spell to take effect. If you try to Siphon from a creature immune to Necrotic damage, the damage falls on you.

Spreading the Love

At 6th level, when any number of living things touch you, you can draw life force from them. The Siphon Damage is evenly distributed among the living things touching you. Any of those living things that successfully save take half damage and the rest is dealt to you. Roll Siphon Damage as usual, but divide by the number of living things touching you (not including yourself) and round up. Excess is still dealt to you.

Got Ink?

Also at 6th level, you learn four spells of your choice from any class. The chosen spells count as Wizard spells for you.

Took You Long Enough

At 10th level, you have discovered some of the secrets of Life and Death. You have managed to ward yourself against outside versions of your own magic. You become immune to Necrotic damage dealt by other creatures. You are still affected by your Siphon Damage, but are unaffected by spells or monsters that deal Necrotic damage. You have also gained enough power to pull energy from anything that has a soul. You can now deal Siphon Damage to any monster that is normally immune to Necrotic damage. If a Construct is described as a "living construct" then you can deal Siphon Damage to it.

I Can Go All Night

Also at 10th level, your work with health siphoning has given you greater stamina and focus. Your Constitution score is no longer limited to a max of 20, and you automatically succeed on maintaining concentration on a spell if you take damage.

Lights Out (Is this a rave?)

At 14th level, you have strengthened your mind to its limit, able to maintain concentration on a spell even when you fall Unconscious.

To Beyond the Grave and Back

Also at 14th level, damage you take while rolling Death Saves does not count for 2 automatic failures. Instead, make a Constitution saving throw against your own Siphon DC. On a fail, only take 1 failure. On a success, no failures.