Most mechanics come from a post from oblivion234. Needs further work. 

Made as a full class with 3 subclass options that can be mixed and matched on the appropriate level. Condinced to a 1 to 10 level system. Attribute points are gained on levels 3,5,7, and 9. Intended for low magic, Gritty Realism, and homebrew rules that make monsters more specialized and dangerous such as vulnerableity and resistances to slashing, bludgeoning, and piercing. 

Trial of the Grasses

Mutation have left you changed forever. 

Mutants

At first level you do a ritual that gives a witcher their enhanced abilities 70% of the people that attempt it are killed in the process. Lucky you survived.

When you get this you gain darkvision (superior dark vision if already optained) , resistance to poison,  increase your speed by 5 ft, proficiency with alchemy tools, increase your life span by 400 years, and +1 to dex, str, and con.

Weapon proficiency: pick between one of the three weapon types to gain proficiency in- piercing (bows, pole arms, etc.), slashing (swords, axes, etc.) or bludgeoning (hammers, maxes, etc.). 

Signs
You gain access to signs which are simple spells that a Witcher can cast. The spells are cantrips but you cannot cast more than one in a row without waiting for 2 more of your turns in combat. They require you to have at least one hand free to cast as well as having a somatic component.

Axii
-A spell that influences your opponent's mind. You can target a creature within 15 ft of you. The target must make a dc 8+your wisdom modifier+ proficiency bonus wisdom saving throw or be stunned for one turn or until damaged by you or one of your allies. If the target is resistant to or immune to charms that applies to this. You cannot apply this to someone already under its effects. This spell requires concentration.
At level 3 this bypasses resistance and immunity
and the DC becomes 10+ your wisdom modifier+ proficientcy bonus .
At level 6 the stun increases to two turns.
At level 8 the stun is no longer broken by taking
damage.

Aard
-A spell that creates a 10x15ft (lengthxwidth) rectangle of force in front of you. This line stops if it hits a creature or structure. A creature in this line must make a DC 8+your wisdom+ proficientcy bonus modifier constitution saving throw or take 1d6 force damage and be knocked prone. A successful save halves this damage and is not knocked prone. Any unsecured items in this area are pushed 10ft backwards.
At level 3 the damage is increased to 2d6.
At level 6 the line now doesn't stop at a creature
and length is increased to 20ft.
At level 8 the damage is increased to 4d6.

Igni
-A spell that causes a cone of fire in front of you. This cone is 10 ft wide and spreads to a length of 15 ft. Creatures caught in this cone must make a DC 8+your wisdom modifier+ proficientcy bonus dex saving throw or take 1d8 fire damage. On a successful save they take half as much. On a failed save roll a D20 if you roll 15 or greater they are also set on fire and take 1d4 fire damage a turn until they use an action to put themselves out.
At level 3 the damage is increased to 2d8.
At level 6 the burning damage increases to 2d6
and you only need to roll a 12 or greater to burn.
At level 8 if the enemy is set on fire their AC
is reduced by 2 for the rest of the fight. This
does not stack.

Quen
-A protective spell that creates a barrier around you. This gives you a 6+your con bonus shield around you represented as temp health. When you take damage enough for the shield to break it explodes in a 5ft circle around you that deals  force damage equal to the health of the shield before the attack. Anyone in the circle must make a DC 8 + your wisdom modifier+ proficientcy bonus dex saving throw. On success they take half as much damage. This shield lasts 2 + your wisdom modifier turns or until it is broken. You can only have one shield active at a time.
At level 3 the shield increases to a 10+your con bonus.
At level 6 the shield increases to 14+your con bonus.
At level 8 the shield increases to 18+your con bonus.

Yrden
-You can create a 15ft circle anywhere within 10ft of you. Any enemy in this circle or that walks into it suffers from extra difficult terrain (1/3 movement), cannot make attacks of opportunity, and each turn they are in the circle they must make a DC 8+ your wisdom modifier+ proficientcy bonus strength save or be unable to move until their next turn and be unable to take bonus actions. If they fail this throw it also counts as loosing concentration for a spell. This circle lasts for 2 rounds. You may only  have one circle active at a time. This spell requires concentration.
At level 3 the circle diameter increases to 20ft.
At level 6 the circle lasts 4 rounds.
At level 8 the diameter increases to 25t.

 

Alchemy
You have a basic understanding of how to make potions. Taking more than 1 potion in one hour causes 1d4+1 unresited poison damage and doubles in damage every additional potion. 

You can spend 10gp worth of materials and 3 hours to make:

A potion of Troll's healing (deals 3 damage to the drinker then heals 1d10+2 hp)
A potion of Falcon's Speed (increases dex by +2 up to 20 for 3 turns deals 7 damage to the drinker)
A potion of Bear Strength (increases strength by +2 up to 20 for 3 turns deals 7 damage to the drinker)    A potion of Cat Eyes (make footprints, sints, and blood glow to you in a 30ft area) 

Your potions can only be used by Witchers due to the harmful ingredients that would be fatal to a non-Witcher.

 

Beastiery

Witcher training has givin you life saving information about monsters. Pick a catagory of monster and all cr 2 and below in that catagory learn their: volnerabilities, resistances, immunities, actions, skills, sinces, languages, lore, biom, str, Dex, con, int, wis, and cha. 

Schools

Each school can be mixed and matched. School options occur on every evem level 2-10. If you wish to pick a different school at a later level than the one picked at level 2 simply gain the lowest level traits you don't already have from a different school instead of the next level in the original school. 

School of the Cat

This school is focused on speed and attack

Attack Verity

At level 4 you gain the ability to use two different types of attacks instead of your normal attack for your action on your turn. Also, gain the ability to use Aard two turns in a row before recharging.

Heavy Strike-You use all your force to channel a blow at without your proficiency but if it strikes you and your allies have advantage to hit for the next round ofattacks on that enemy

Whirling Blows-You can hit up to three targets within 5 feet of you. Roll each attack separately. You do not get to add your proficiency bonus to hit.

Extra attack

At level 6 gain an extra attack action and crit on 18-20

Feline Finesse

At level 2 when you take this school you now land a critical hit on a 19 or 20. Gain +1 AC in light armor. Also, gain proficiency in acrobatics and the ability to use acrobatics and athletics as a bonus action in combat in order to mount or grapple an enemy. 

School of the Viper

This class focuses on knowing the enemy and alchemy. 

Brewer

At level 6 you are the master of reading your enemy and knowing their weaknesses. They now have disadvantage when rolling against your specialized poison and after you have used hunters eye on them as long as an ally is within 5 feet of that enemy you have advantage on your attack rolls against them. Gain knowledge about the two categories of creatures already know up to CR 4.

Also, gain specialized potions the can be created using the same rules as your other potions:

Potion of Swallow- gain 3d4+ Int mod temporary hit points for 1 min

Potion of the Viper- Gain an extra bonus action for 1 minute. 

Bomb of Dragon's Breath- Throw this creation 15/30 ft to cause a 10ft fire damage explosion circle with a DC Dex save equal to 8+ Prof mod+ Int mod. On a fail take 2D10 fire damage and ignite the area and creatures on fire for 1 min dealing 1d4+ Int mod fire damage. On a success take half damage and no damage over time. 

Hunters Eye

At level 4 you are specifically good at identifying your  enemy's weaknesses. Use hunter's eye as an action to identify a resistance or vulnerableity you don't already know and gain advantage on your next attack against that target.

Poison Master

At level 2 when you choose this school you gain the ability to make poison for your blade called blade oil. It costs 10gp worth of materials to make and can be applied 3 times with one bottle. It lasts 1 min while applied.
Make specific blade oil based on an enemies weaknesses. Make an oil of any an resistant or vulnerableity. Oil makes your attacks ignore the specified resistance. If the enemy is vulnerable they must make a DC 8+INT mod+ proficiency bonus con saving throw at disadvantage or they are poisoned and take 2d6 poison damage for 1 min. If the enemy has no resistance or vulnerableity make a poison with DC 8+INT mod+ proficiency bonus con saving throw with 2d4 poison damage for 1 min. 

Also, gain knowledge of a category of monsters CR 3 and below and increase knowledge of monsters already known to CR 3.

School of the Griffin

This school is focused on signs.

Focused Signs

At level 4 enemies gain disadvantage one checks against your signs. Also if the sign deals damage add your Int mod to that damage.

Also, you can now cast signs as a bonus action. 

Keen Mind

When you choose this school at level 2. You gain advantage on concentration check for Quen, Yrden, and Axii. 

Also, signs require one less turn before you can use another.

Power Signs

At level 6 your signs now have an additional effect.
Axii-Instead of stunning you may instead choose to turn the target into an ally for two turns or they take damage.
Aard-The area is now a circle with a 15 ft radius around you.
Igni-If the saving roll is failed they can ignite their allies within 5ft of them for 1 min if the fail the same save.
Quen-You may cast Quen as a reaction to a successful hit but it is half as powerful and they are pushed back 10ft.
Yrden-Enemies in the circle now take 2d10 worth of thunder damage every round they are in the circle.

Also, you have no cooldown between signs.