Witchers are humans who since childhood undergo extensive training, mental and physical conditioning as well as passed through the extremely dangerous - with about 25% survival rate - alchemical and mutagenic process that greatly enhances their abilities. Geralt is a bit special because he was the only child that survived the advanced mutations (most likely because his mother was an enchantress).
The abilities of witchers are mostly physical:
- Enhanced senses: Witchers can identify the animal species from the smell of its blood, hear a heartbeat of nearby creatures, but the most impressive are their cat-like eyes, that allow seeing in near-absolute darkness or notice minuscule details of the surrounding.
- Resistance to toxic substances: witchers can further enhance their abilities using special elixirs that are normally toxic to humans. They can also resist many poisonous attacks, that would normally kill an ordinary human in seconds.
- Enhanced speed, reflexes, endurance and to a bit lesser extent strength, far beyond any normal or well-trained human, that allows them to swiftly end fights with minimal effort, and perform physical feats non-witchers couldn't hope to match.
- Much faster healing rate and near immunity to diseases
- Greatly extended longevity - while no Witcher ever died from the old age, it has been speculated that the oldest living witcher - Vessemir - can be as much as ~300 years old, but he appears to be around 60.
There are a few, probably unintentional side effects such as complete sterility. Few enchantresses also mentioned that even an innocent touch of a witcher can bring a pleasant tingling sensation.
The magic of the witchers pales in comparison to their physical skills - they don't know any spectacular magic, although they know a few basic spells that they call "signs":
The so-called Sign of Aard, Ciri, is a very simple spell belonging to the family of psychokinetic magic which is based on thrusting energy in the required direction. The force of the thrust depends on how the will of the person throwing it is focused and on the expelled force. It can be considerable. The witchers adapted the spell, making use of
the fact that it does not require knowledge of a magical formula—concentration and the gesture are enough. That’s why they called it a Sign. Sign. Where they got the name from, I don’t know, maybe from the Elder Speech – the word ‘ard’, as you know, means ‘mountain’, ‘upper’ or ‘the highest’. If that is truly the case then the name is very misleading because it’s hard to find an easier psychokinetic spell. We, obviously, aren’t going to waste time and energy on something as primitive as the witchers’ Sign.
The power of the signs also pales in comparison to power of trained mages - there is a scene somewhere in the books where Yennefer says, that using the same technique as the Aard sign, she could send a rock to the moon - in comparison, Geralt can "barely" use it to knock someone off their feet.
There are a few known signs (there were not greatly explained in the books but the game series explained them nicely):
- Aard is a psychokinetic push
- Igni allows creating a small stream of fire
- Yrden is a magical trap briefly immobilizing opponent
- Quen created a small magical barrier
- Axii was allowing to briefly charm someone or calm a panicked horse
The books and games also mention the sign of Heliotrop in a few places, but its purpose is inconsistent - i.e. onece it is described as an offensive sign, once as protection from magic and once it is used to put someone to sleep.
Also, contrary to wizards, witchers don't take the magic from their surroundings/primal Chaos/elements, but instead, they fuel it with elixirs.
