You underwent, whether by your own decision or force, the Witcher Trials, and are now a mutated form of whatever race you were, granting you sharper senses, and better skill with your weapons, as well as a simple form of magic known as Rune Magic. 

Rune Magic

Starting at 3rd level, you can cast Rune magic due to your Witcher mutations. Your spellcasting modifier is Constitution 

You can cast the following spells as if they were 3rd level: Thunderwave, Burning Hands, Suggestion, Shield, Magic Circle. You can cast these spells with only Somatic components. You can only cast one spell per turn, and all spells are made into bonus actions whenever you cast them using Rune Magic. You can use this feature to cast 3 spells per short rest

Trained Swordsmanship

Starting when you take this Archetype at 3rd level, When you are attacked by an opponent's melee weapon, you can use your reaction to raise your AC by your constitution modifier. If this causes the attack to miss, you can attempt an attack. You make the attack roll with advantage if your AC was 2 more than the opponent's attack roll.

Witcher Senses

At 7th level, your mutations have grow stronger and now your senses have heightened.

You have advantage on intelligence(Investigation) checks made to find tracks, remains, or objects. You also have advantage on wisdom(Perception) checks requiring sight

Practiced Potions

 At 10th level, you learn the alchemical science of potion making.

You can spend an hour creating 1 of 3 types of potions. This can happen during a long or short rest, and doesn't require any components. You can make a standard Healing Potion, a potion of Hill Giant's Strength, or an Oil of Slipperiness

Enhanced Technique

By 15th level, your technique with your sword has grown and you now know one of the Witchers' less known techniques.

You crit on 18-20 for attack rolls. You can also use your action to make a spin attack. In that case, make separate attack rolls for all creatures within your weapons reach. The Spin attack deals extra damage equal to 2d8+proficiency bonus. This attack can be used once per turn.

Master Witcher

By 18th level, all of your Witcher training and technique has reached mastery, and all of your abilities are affected in one way or another by your mutations and skills.

Your Rune Magic spells are now cast at 6th level, your spin attack can use one of the Rune charges to deal an extra 1d8 damage and get 5 ft more reach, your Witcher Senses give you Expertise in wisdom(Perception) checks, and intelligence(Investigation) checks, you can now make 2 potions per short rest, and they can now be Greater Healing, Frost Giant Strength, Potion of Speed, or Oil of Sharpness