A Witcher has the potential to live up to 500 years old, but most of them are killed before they reach 200. 

Most Witchers are neutral on principle, but there are known to be extremes. Witchers such as those from the School of the Cat tend towards chaos and evil, where as those from the School of the Bear are generally more reclusive and lawful. Few are known to be altruistic. 

Your Strength, Dexterity, and Constitution scores increase by 1.

The Witcher Trials have left your eyes yellow and cat-like.

You gain Darkvision up to 60ft. 

Your Witcher biology gives an extreme resistance to diseases, along with a superior resistance to poisons.

You gain immunity to diseases.
You gain resistance to poison damage.
You gain advantage on Constitution saving throws to resist the Poisoned condition.

Your sense of smell, sight, hearing, and taste are all incredibly enhanced.

You gain Blindsight out to 10ft. You lose this sense while you are deafened. 
Your superior sight gives you advantage on Survival checks to locate tracks from other creatures. 
Your sense of smell allows you to determine a creature's type simply by smelling their blood as an action. 

Your body is hardier than that of a normal humanoid.

You have advantage on your first three death saving throws.
When regaining hit points, roll 1d4 and add the result to the amount you heal.

You gain the use of the five Witcher signs.

Aard - Gain the use of the Aard Sign, which you can cast as an action. Any creature in a 15ft cone takes 1d6 force damage and must make a Strength saving throw (DC 14). If they fail, they are also pushed 15ft away from you in a straight line. This damage increases by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 17th level (4d6).
Quen - As a bonus action, gain 1d8 temporary hit points. The number of temporary hit points increases by 1d8 when you reach 5th level (2d8), 11th level (3d8), and 17th level (4d8).
Igni - Gain use of the Produce Flame cantrip
Axii - Gain the use of Charm Person, which you can cast at 1st level at will. When cast this way, the duration of the spell is 10 minutes and you may only have one creature affected by Charm Person at a time. 
Yrden - As a bonus action, choose any point within 10ft of you. A 15ft sphere appears in that location for 1 minute. When an enemy ends their turn within the sphere, their movement speed is halved, and any creatures under the effects of "Incorporeal Movement" are forced to end this ability. These effects last until they end their turn outside of the Yrden trap. 

Additionally, gain the Prestidigitation cantrip.

Constitution is your spellcasting ability for these spells. 

Given to Witchers who survive the Trial of the Grasses, a Witcher's Medallion is one of a Witcher's most important assets, along with their steel and silver swords. A Witcher will not part with their medallion voluntarily, usually refusing to ever take it off. 

Add a Witcher Medallion to your inventory. Its appearance is dictated by the Witcher School trait.
When within 100ft to any kind of magic or magical being with a challenge rating below 6, the medallion will vibrate, alerting you to its presence. You do not learn the specific location of the source, only that the medallion will vibrate more intensely when closer to the source of the magic.
Additionally, no aberration, fey, fiend, monstrosity, or undead creature may consciously and willingly make physical contact with the medallion. 

The medallion can be lost or stolen. They are also incredibly difficult to destroy, but may be destroyed by intense heat or being submerged in acid for an extended period, or any other method at DM's discretion. If this happens, remove the medallion from your inventory, and lose the ability to detect magic this way until you can replace it. 

You can speak Common, and two other languages of your choice. 

You have undergone rigorous physical and mental training. Choose the Witcher School you trained at. 

Gain proficiency in Athletics, Nature, and Survival skill checks. 

While being able to sleep, Witchers do not need it. Instead, they are able to meditate for four hours a day, remaining semiconscious and aware of their surroundings while doing so. While meditating, you can still perceive your surroundings normally, but cannot interact with anything without interrupting the meditation and sacrificing the benefits of your rest while doing so. After meditating in this way, you gain the same benefits a human would gain from eight hours of sleep.